Drag 'n Drop Inline Images Backend - Form

Part of a set of changes to add support for dragging and dropping images into
the markdown editor.

Adding a form to create FileAttachments owned by a user. Moving the mimetype
processing stuff to the mimetypes.py file. Adding tests around FileAttachments
owned by users.

Testing Done:
All unit tests pass after making the changes.

Added unit tests around the changes to the FileAttachment model:
- Tests around creating a FileAttachment with/without a file
- Tests around the is_accessible_by(user) and is_mutable_by(user) methods

Reviewed at https://reviews.reviewboard.org/r/6615/