Handle unknown filetypes and report them to user

This fixes a regression that was introduced because some file-types were supported (svg and eot) that got removed in the move over to mime magic. Adds tests to verify those file-types still work. This also adds some logging that will report unknown filetypes to the user so they know that a file we couldn't figure out was upload as binary.

Followup on #104 

Please review @maartenvg 
add ```MimeMagic.add('application/json', extensions: %w(json))``` to that for JSON