Add a files pattern?

[lundberg]

As mentioned in #87, it might be of interest to allow matching a request on files being sent.

• What do we want to match on?
• Is it even possible once internal HTTPX request is built.

route = respx.post("https://example.org/", files=?)

Since uploading files with HTTPX supports multiple arg types, it might only be possible to match on file names, if given. Reading a file object's content could affect HTTPX internals, if not handled properly, like reseting seek point etc.

Needs further investigation, multiple files.