Recent changes in thumbnail generators revealed that finfo module cannot
be relied on - I found a GIF file that started with usual "GIF87a", and
yet finfo reported its mime type to be "application/octet-stream".
This change removes reliance on finfo module and implements detecting
common file types supported by szurubooru in manual fashion. This might,
of course, introduce some bugs that weren't present when using finfo.
However, this change was made exactly because of such bug - while
previously such edge cases were impossible to fix, now they can be
dealt with accordingly in our own code.
Also from what I saw on the Internet, getting fileinfo to work on
Windows is a lot of trouble, so this change is welcome from that angle,
too.
