Fixed bug 561 Improve mimetype

Had an interview with DG yesterday and figured I might as well submit a code sample in the form of a bug fix. 
I couldn't figure out how to get Islandora to actually use this class, but have tested it externally which should be fine as it is a pretty simple class. As long as the file_default_mimetype_mapping function and watchdog functions exist in Drupal, it should work fine. 

Bug:
https://jira.duraspace.org/browse/ISLANDORA-561