Currently we have no centralized method for deleting extra files associated

with file attachments, such as thumbnail files. This adds a

delete_associated_files method to the base MimetypeHandler class, which

can be implemented by subclasses to take care of deleting any extra associated

files that are created for file attachments of certain mimetypes. This also

updates the ImageMimetype class to delete the files it creates for thumbnails.

Tested in an upcoming change where deleting draft file attachments will

  trigger deleting all files associated with the draft.
Ran unit tests.