Guess MIME types for objects uploaded as application/octet-stream

Generally, if a browser doesn't know what MIME type a file is, it will upload it as application/octet-stream. This can cause problems if one person uploads a file that their browser doesn't understand, then another browser tries to download it and doesn't understand what kind of file it's getting.

If a browser uploads a file with MIME type application/octet-stream, we should use mime.types-style guessing to guess the content-type from the file extension.

There should be a mechanism for site admins to add new extension-MIME mappings (preferably through the web UI, but editing a file would work OK)

If a browser specifies a MIME type of an uploaded file as anything but application/octet-stream, we trust the browser.