HTTP file uploading standard?

I'm writing a client in python to upload a file to a webserver I run. On the PHP side, it seems I should expect the file to arrive in the $_FILES variable, but I can't find any documentation on how that variable gets populated. I can find tutori开发者_C百科als on how to create web forms to upload, but none of them tell me what the actual HTTP POST looks like. I'm assuming that there is a standard for what parameters to populate and what content-type to specify, but I can't find it stated anywhere.

It's the multipart/form-data encoding which is described in RFC 1867 and RFC 2388.