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.
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
When you are writing client-side code:
multipart/form-datawhen your form includes any
- otherwise you can use
application/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).