Ability to disable mimetype-guessing for send_file

[Markus Unterwaditzer]

The file has no file extension, but I know the mimetype.

Some ideas:

• Use set_header_fallback inside send_file so I can set the header beforehand
• Implement Responder for file objects, change send_file to build the mimetype guessing on top of that.

[Kevin Butler]

@untitaker The first suggestion sounds worth doing regardless, would you like to PR that change?

I've considered implementing Responder for File/Path before, but I can't remember what was iffy about that, it might be worth investigating again :)