What does enctype='multipart/form-data' mean?

Question

What does enctype='multipart/form-data' mean in an HTML form and when should we use it?

Answer 1

When you make a POST request, you have to encode the data that forms the body of the request in some way.

HTML forms provide three methods of encoding.

• application/x-www-form-urlencoded (the default)
• multipart/form-data
• text/plain

Work was being done on adding application/json, but that has been abandoned.

(Other encodings are possible with HTTP requests generated using other means than an HTML form submission. JSON is a common format for use with web services and some still use SOAP.)

The specifics of the formats don't matter to most developers. The important points are:

• Never use text/plain.

When you are writing client-side code:

• use multipart/form-data when your form includes any <input type="file"> elements
• otherwise you can use multipart/form-data or application/x-www-form-urlencoded but application/x-www-form-urlencoded will be more efficient

When you are writing server-side code:

• Use a prewritten form handling library

Most (such as Perl's CGI->param or the one exposed by PHP's $_POST superglobal) will take care of the differences for you. Don't bother trying to parse the raw input received by the server.

Sometimes you will find a library that can't handle both formats. Node.js's most popular library for handling form data is body-parser which cannot handle multipart requests (but has documentation which recommends some alternatives which can).

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If you are writing (or debugging) a library for parsing or generating the raw data, then you need to start worrying about the format. You might also want to know about it for interest's sake.

application/x-www-form-urlencoded is more or less the same as a query string on the end of the URL.

multipart/form-data is significantly more complicated but it allows entire files to be included in the data. An example of the result can be found in the HTML 4 specification.

text/plain is introduced by HTML 5 and is useful only for debugging — from the spec: They are not reliably interpretable by computer — and I'd argue that the others combined with tools (like the Network Panel in the developer tools of most browsers) are better for that).