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-datatext/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-datawhen your form includes any<input type="file">elements - otherwise you can use
multipart/form-dataorapplication/x-www-form-urlencodedbutapplication/x-www-form-urlencodedwill 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 that recommends some alternatives which can).
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).