Make sure content-type override also works on files

[TheWolfNL]

Before:

• If you POST a file it would always use the content type derived from the file, even when a content type is supplied.

After:

• If no content type is given, the default is ensured in src/url.c (lines 357 and 637)
• If no content type is given, but a file is POSTed, the file's content type will be used
• If a content type is given, and a file is POSTed, the given content type will be used

This solves:
#105
and would allow people posting a file but the content type is guessed incorrectly to easily workaround the problem by forcing the desired content type.

[TheWolfNL]

@JoeDog It would be great if you could merge this one as well.