Enhanced upload file type detection. If given an original filename, we'll attempt...

Enhanced upload file type detection. If given an original filename, we'll attempt to detect type from the extension if we were unable to make a definitive match from content. Generic octet-stream, zip, and MS Office type are explicitly singled out for re-checks, which fixes OpenOffice and MS Office documents to come up with the proper types when misdetected.
File extensions can also be added to the upload type whitelist; they'll be normalized to types for the actual comparison, so only known extensions will work.