Currently, when file attachments get completely deleted from the database we

keep their associated files lying around. There's no good reason to do this,

when the file attachment gets deleted we have no way of accessing these files

anymore.
This change deletes associated files from the filesystem when file

attachments get deleted. Any files created by the MimetypeHandler will be

deleted, and then the main file of the file attachment will be deleted.

Tested deleting draft .png file attachments that have thumbnail files,

  saw that they were deleted.
Tested deleting draft .txt files.
Tested deleting published files, saw that their files weren't deleted.
Tested deleting a draft revision of a file attachment that has

  been published before (has a history), saw that only the files for

  the draft revision were deleted.