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.

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