Internet draft for 'file upload' feature proposal

Larry Masinter (
Sat, 12 Nov 94 17:51:38 EST

This internet draft is for consideration by the HTML working group for
inclusion in HTML.


File Transmission from WWW Browsers to Servers L. Masinter
draft-ietf-html-fileupload-00.txt Xerox Corporation
Expires May 18, 1995 November 18, 1994

File Transfer from World-Wide Web Browsers to Servers

i. Status of this Memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''

To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on (Africa), (Europe), (Pacific Rim), (US East Coast), or (US West Coast).

1. Abstract

Currently, a World-Wide Web server can get information from users
with HTML forms. These forms have proven useful in a wide variety
of applications in which input from the user is necessary. But this
capability is still greatly limited because HTML forms don't provide
a way for the user to submit files to the server. Service providers
who need to get files from the user have had to implement custom
browsers. (Examples of these custom browsers have appeared on the
www-talk mailing list.) To avoid the necessity for custom browsers
and to make WWW servers complete in their ability to get information
from the user, the WWW needs to provide a way for users to send files
to servers. Since user information is sent back to the server using
HTML forms, it is most logical to extend HTML forms to support file

This document proposes an extention to HTML to allow forms to request
users supply files as data to be returned when the form has been
completely filled out and submitted. It also includes a description of
a backward compatibility strategy that allows new servers to interact
with old WWW browsers.

2. HTML forms with file submission

The current draft HTML specification <URL:
%7Fconnolly/html-spec/spyglass-19941014/html-19941014.txt.Z> defines
eight possible values for the attribute TYPE of an INPUT element:

In addition, it defines the default ENCTYPE attribute of the FORM
element using the POST METHOD to have the default type

This proposal makes two changes:
1) add a FILE option for the TYPE attribute of INPUT
2) allow the ENCTYPE of a FORM to be "multipart/www-form-data".

These changes might be considered independently, but are both
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/www-form-data" ACTION="_URL_" METHOD=POST>

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

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


The change to the HTML DTD is trivial--just one item added to the
entity "InputType", as follows:

... (other elements) ...

TYPE %InputType TEXT
NAME CDATA #IMPLIED -- required for all but submit and reset
SRC %URI #IMPLIED -- for image inputs --
but delimited with comma, not space
ALIGN (top|middle|bottom) #IMPLIED

... (other elements) ...

This is the minimal change requested. Other, larger changes to the
InputType entity might also be contemplated but are not part of this
proposal. For example, an INPUT element might usefully have an
attribute which identifies a set of acceptable media-types, e.g.,
<INPUT TYPE=file ACCEPT="image/gif, image/tiff" NAME="image1">.

3. Proposed implementation

The proposed implementation in WWW browsers is, when a INPUT tag of
type FILE is encountered, to show a 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 pops 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.

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 efficiently sending large quantities of binary data.
Thus, a (new) media type, "multipart/www-form-data" is proposed as a
way of efficiently sending the values associated with a filled-out
form from client to server.

The media-type (MIME-type) "multipart/www-form-data" follows the
rules of all multipart MIME data streams as outlined in RFC 1521--a
boundary is selected that does not occur (with more than
infinitessimal probability) in any of the data. 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 using a "Name: " attribute. Each part has an optional
Content-Type (which defaults to text/plain). File inputs should be
identified as either application/binary or the appropriate media
type, if known. If multiple files were selected, they should be
transferred together using the multipart/mixed format. The
"content-transfer-encoding" for each part should be "binary".

File inputs may optionally identify the file name using the
"Content-Description" header. Browers may optionally include a
Content-Length header both in the overall reply and in individual
components; the content-length is not intended as a replacement for
the multipart boundary as a way of detecting the end of an
individual component; rather, it is just as a way forwarning the
server of the amount of data coming.

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/www-form-data, parse the
various fields (checking for validity, writing the file data to local
files for subsequent processing, etc.).

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
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/www-form-data,
and know that the user was using a browser that didn't implement file

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 that
* 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.

This data stream would be marked as application/x-please-send-files.

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

5. Other considerations


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.

Deferred file transmission:

In some situations, it might be advisable to have the server validate
various elements of the clients 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. 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.

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, trivial to implement on
both the sending client and receiving server, and as efficient as
other methods of dealing with multiple combinations of binary data.

Not overloading <INPUT>:

Various people have wondered about the advisability of overloading
'INPUT' for this function, rather than merely providing a different
type of FORM element. Among other considerations, the migration
strategy which is allowed when using <INPUT> is important. In
addition, the <INPUT> field *is* already overloaded to contain most
kinds of data input; rather than creating multiple kinds of <INPUT>
tags, it seems most reasonable to enhance <INPUT>. The 'type' of
INPUT is not the content-type of what is returned, but rather the
'widget-type'; i.e., it identifies the interaction style with the
user. The description here is carefully written to allow <INPUT
TYPE=FILE> to work for text browsers or audio-markup.

Default content-type of field data:

Many input fields in HTML are to be typed in. There has been some
ambiguity as to how form data should be transmitted back to servers.
Making the content-type of <INPUT> fields be text/plain clearly
disambiguates that the client should properly encode the data before
sending it back to the server with CRLFs.

6. Examples

Suppose the server supplies the following HTML:

<FORM ACTION="http://server.dom/cgi/handle">
What is your name? <INPUT TYPE=TEXT NAME=submitter>
What files are you sending? <INPUT TYPE=FILE NAME=pics>

and the user types "Joe Blow" in the name field, and selects
a text file "file1.txt" and also an image file "file2.gif" for
the answer to 'What files are you sending?'.

The client would send back the following data:

Content-type: multipart/www-form-data, boundary=AaB03x
Name: field1

Joe Blow
Name: pics
Content-type: multipart/mixed, boundary=BbC04y

Content-Description: file1.txt
Content-Type: text/plain

... contents of file1.txt ...
Content-Description: file2.gif
Content-type: image/gif

...contents of file2.gif...

7. Conclusion

The suggested implementation gives the client a lot of flexibility in
the number and types of files it can send to the server, it gives the
server control of the decision to accept the files, and it gives
servers a chance to interact with browsers which do not support INPUT
TYPE "file".

The change to the HTML DTD is very simple, but very powerful. It
enables a much greater variety of services to be implemented via the
World-Wide Web than is currently possible due to the lack of a file
submission facility. This would be an extremely valuable addition to
the capabilities of the World-Wide Web.

A. Authors' Addresses

Larry Masinter
Xerox Palo Alto Research Center Voice: (415) 812-4365
3333 Coyote Hill Road Fax: (415) 812-4333
Palo Alto, CA 94304

Ernesto Nebel
XSoft, Xerox Corporation Voice: (619) 676-7817
10875 Rancho Bernardo Road, Suite 200 Fax: (619) 676-7865
San Diego, CA 92127-2116