rfc1867.txt

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Network Working Group                                           E. Nebel
Request For Comments: 1867                                   L. Masinter
Category: Experimental                                 Xerox Corporation
                                                           November 1995


                     Form-based File Upload in HTML

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

1. Abstract

   Currently, HTML forms allow the producer of the form to request
   information from the user reading the form.  These forms have proven
   useful in a wide variety of applications in which input from the user
   is necessary.  However, this capability is limited because HTML forms
   don't provide a way to ask the user to submit files of data.  Service
   providers who need to get files from the user have had to implement
   custom user applications.  (Examples of these custom browsers have
   appeared on the www-talk mailing list.)  Since file-upload is a
   feature that will benefit many applications, this proposes an
   extension to HTML to allow information providers to express file
   upload requests uniformly, and a MIME compatible representation for
   file upload responses.  This also includes a description of a
   backward compatibility strategy that allows new servers to interact
   with the current HTML user agents.

   The proposal is independent of which version of HTML it becomes a
   part.

2.  HTML forms with file submission

   The current HTML specification defines eight possible values for the
   attribute TYPE of an INPUT element: CHECKBOX, HIDDEN, IMAGE,
   PASSWORD, RADIO, RESET, SUBMIT, TEXT.

   In addition, it defines the default ENCTYPE attribute of the FORM
   element using the POST METHOD to have the default value
   "application/x-www-form-urlencoded".







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   This proposal makes two changes to HTML:

   1) Add a FILE option for the TYPE attribute of INPUT.
   2) Allow an ACCEPT attribute for INPUT tag, which is a list of
      media types or type patterns allowed for the input.

   In addition, it defines a new MIME media type, multipart/form-data,
   and specifies the behavior of HTML user agents when interpreting a
   form with ENCTYPE="multipart/form-data" and/or <INPUT type="file">
   tags.

   These changes might be considered independently, but are all
   necessary for reasonable file upload.

   The author of an HTML form who wants to request one or more files
   from a user would write (for example):

    <FORM ENCTYPE="multipart/form-data" ACTION="_URL_" METHOD=POST>

    File to process: <INPUT NAME="userfile1" TYPE="file">

    <INPUT TYPE="submit" VALUE="Send File">

    </FORM>

   The change to the HTML DTD is to add one item to the entity
   "InputType". In addition, it is proposed that the INPUT tag have an
   ACCEPT attribute, which is a list of comma-separated media types.

  ... (other elements) ...

  <!ENTITY % InputType "(TEXT | PASSWORD | CHECKBOX |
                         RADIO | SUBMIT | RESET |
                         IMAGE | HIDDEN | FILE )">
  <!ELEMENT INPUT - 0 EMPTY>
  <!ATTLIST INPUT
          TYPE %InputType TEXT
          NAME CDATA #IMPLIED  -- required for all but submit and reset
          VALUE CDATA #IMPLIED
          SRC %URI #IMPLIED  -- for image inputs --
          CHECKED (CHECKED) #IMPLIED
          SIZE CDATA #IMPLIED  --like NUMBERS,
                                  but delimited with comma, not space
          MAXLENGTH NUMBER #IMPLIED
          ALIGN (top|middle|bottom) #IMPLIED
          ACCEPT CDATA #IMPLIED --list of content types
          >




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  ... (other elements) ...

3.  Suggested implementation

   While user agents that interpret HTML have wide leeway to choose the
   most appropriate mechanism for their context, this section suggests
   how one class of user agent, WWW browsers, might implement file
   upload.

3.1 Display of FILE widget

   When a INPUT tag of type FILE is encountered, the browser might show
   a display of (previously selected) file names, and a "Browse" button
   or selection method. Selecting the "Browse" button would cause the
   browser to enter into a file selection mode appropriate for the
   platform. Window-based browsers might pop up a file selection window,
   for example. In such a file selection dialog, the user would have the
   option of replacing a current selection, adding a new file selection,
   etc. Browser implementors might choose let the list of file names be
   manually edited.

   If an ACCEPT attribute is present, the browser might constrain the
   file patterns prompted for to match those with the corresponding
   appropriate file extensions for the platform.

3.2 Action on submit

   When the user completes the form, and selects the SUBMIT element, the
   browser should send the form data and the content of the selected
   files.  The encoding type application/x-www-form-urlencoded is
   inefficient for sending large quantities of binary data or text
   containing non-ASCII characters.  Thus, a new media type,
   multipart/form-data, is proposed as a way of efficiently sending the
   values associated with a filled-out form from client to server.

3.3 use of multipart/form-data

   The definition of multipart/form-data is included in section 7.  A
   boundary is selected that does not occur in any of the data. (This
   selection is sometimes done probabilisticly.) Each field of the form
   is sent, in the order in which it occurs in the form, as a part of
   the multipart stream.  Each part identifies the INPUT name within the
   original HTML form. Each part should be labelled with an appropriate
   content-type if the media type is known (e.g., inferred from the file
   extension or operating system typing information) or as
   application/octet-stream.





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   If multiple files are selected, they should be transferred together
   using the multipart/mixed format.

   While the HTTP protocol can transport arbitrary BINARY data, the
   default for mail transport (e.g., if the ACTION is a "mailto:" URL)
   is the 7BIT encoding.  The value supplied for a part may need to be
   encoded and the "content-transfer-encoding" header supplied if the
   value does not conform to the default encoding.  [See section 5 of
   RFC 1521 for more details.]

   The original local file name may be supplied as well, either as a
   'filename' parameter either of the 'content-disposition: form-data'
   header or in the case of multiple files in a 'content-disposition:
   file' header of the subpart. The client application should make best
   effort to supply the file name; if the file name of the client's
   operating system is not in US-ASCII, the file name might be
   approximated or encoded using the method of RFC 1522.  This is a
   convenience for those cases where, for example, the uploaded files
   might contain references to each other, e.g., a TeX file and its .sty
   auxiliary style description.

   On the server end, the ACTION might point to a HTTP URL that
   implements the forms action via CGI. In such a case, the CGI program
   would note that the content-type is multipart/form-data, parse the
   various fields (checking for validity, writing the file data to local
   files for subsequent processing, etc.).

3.4 Interpretation of other attributes

   The VALUE attribute might be used with <INPUT TYPE=file> tags for a
   default file name. This use is probably platform dependent.  It might
   be useful, however, in sequences of more than one transaction, e.g.,
   to avoid having the user prompted for the same file name over and
   over again.

   The SIZE attribute might be specified using SIZE=width,height, where
   width is some default for file name width, while height is the
   expected size showing the list of selected files.  For example, this
   would be useful for forms designers who expect to get several files
   and who would like to show a multiline file input field in the
   browser (with a "browse" button beside it, hopefully).  It would be
   useful to show a one line text field when no height is specified
   (when the forms designer expects one file, only) and to show a
   multiline text area with scrollbars when the height is greater than 1
   (when the forms designer expects multiple files).






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4.  Backward compatibility issues

   While not necessary for successful adoption of an enhancement to the
   current WWW form mechanism, it is useful to also plan for a migration
   strategy: users with older browsers can still participate in file
   upload dialogs, using a helper application. Most current web browers,
   when given <INPUT TYPE=FILE>, will treat it as <INPUT TYPE=TEXT> and
   give the user a text box. The user can type in a file name into this
   text box. In addition, current browsers seem to ignore the ENCTYPE
   parameter in the <FORM> element, and always transmit the data as
   application/x-www-form-urlencoded.

   Thus, the server CGI might be written in a way that would note that
   the form data returned had content-type application/x-www-form-
   urlencoded instead of multipart/form-data, and know that the user was
   using a browser that didn't implement file upload.

   In this case, rather than replying with a "text/html" response, the
   CGI on the server could instead send back a data stream that a helper
   application might process instead; this would be a data stream of
   type "application/x-please-send-files", which contains:

   * The (fully qualified) URL to which the actual form data should
     be posted (terminated with CRLF)
   * The list of field names that were supposed to be file contents
     (space separated, terminated with CRLF)
   * The entire original application/x-www-form-urlencoded form data
     as originally sent from client to server.

   In this case, the browser needs to be configured to process
   application/x-please-send-files to launch a helper application.

   The helper would read the form data, note which fields contained
   'local file names' that needed to be replaced with their data
   content, might itself prompt the user for changing or adding to the
   list of files available, and then repackage the data & file contents
   in multipart/form-data for retransmission back to the server.

   The helper would generate the kind of data that a 'new' browser
   should actually have sent in the first place, with the intention that
   the URL to which it is sent corresponds to the original ACTION URL.
   The point of this is that the server can use the *same* CGI to
   implement the mechanism for dealing with both old and new browsers.

   The helper need not display the form data, but *should* ensure that
   the user actually be prompted about the suitability of sending the
   files requested (this is to avoid a security problem with malicious
   servers that ask for files that weren't actually promised by the



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   user.) It would be useful if the status of the transfer of the files
   involved could be displayed.

5.  Other considerations

5.1 Compression, encryption

   This scheme doesn't address the possible compression of files.  After
   some consideration, it seemed that the optimization issues of file
   compression were too complex to try to automatically have browsers
   decide that files should be compressed.  Many link-layer transport
   mechanisms (e.g., high-speed modems) perform data compression over
   the link, and optimizing for compression at this layer might not be
   appropriate. It might be possible for browsers to optionally produce
   a content-transfer-encoding of x-compress for file data, and for
   servers to decompress the data before processing, if desired; this
   was left out of the proposal, however.

   Similarly, the proposal does not contain a mechanism for encryption
   of the data; this should be handled by whatever other mechanisms are
   in place for secure transmission of data, whether via secure HTTP or
   mail.

5.2 Deferred file transmission

   In some situations, it might be advisable to have the server validate
   various elements of the form data (user name, account, etc.)  before
   actually preparing to receive the data.  However, after some
   consideration, it seemed best to require that servers that wish to do
   this should implement this as a series of forms, where some of the
   data elements that were previously validated might be sent back to
   the client as 'hidden' fields, or by arranging the form so that the
   elements that need validation occur first.  This puts the onus of
   maintaining the state of a transaction only on those servers that
   wish to build a complex application, while allowing those cases that
   have simple input needs to be built simply.

   The HTTP protocol may require a content-length for the overall
   transmission. Even if it were not to do so, HTTP clients are
   encouraged to supply content-length for overall file input so that a
   busy server could detect if the proposed file data is too large to be
   processed reasonably and just return an error code and close the
   connection without waiting to process all of the incoming data.  Some
   current implementations of CGI require a content-length in all POST
   transactions.

   If the INPUT tag includes the attribute MAXLENGTH, the user agent
   should consider its value to represent the maximum Content-Length (in



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   bytes) which the server will accept for transferred files.  In this
   way, servers can hint to the client how much space they have
   available for a file upload, before that upload takes place.  It is
   important to note, however, that this is only a hint, and the actual
   requirements of the server may change between form creation and file
   submission.

   In any case, a HTTP server may abort a file upload in the middle of
   the transaction if the file being received is too large.

5.3 Other choices for return transmission of binary data

   Various people have suggested using new mime top-level type
   "aggregate", e.g., aggregate/mixed or a content-transfer-encoding of
   "packet" to express indeterminate-length binary data, rather than
   relying on the multipart-style boundaries.  While we are not opposed
   to doing so, this would require additional design and standardization
   work to get acceptance of "aggregate".  On the other hand, the
   'multipart' mechanisms are well established, simple to implement on
   both the sending client and receiving server, and as efficient as
   other methods of dealing with multiple combinations of binary data.

5.4 Not overloading <INPUT>: