Added two more options: delete image-only attachments; delete previews (like oembed thumbs)
Some further minor improvements.
Thanks to colegota for spotting this issue.
The code was so involved there was even a comment asking for a refactor.
Now, File_redirection::where always returns a nice File_redirection
object instead of an array or string or nothing. The object is
either one which already existed or else a new, unsaved object.
Instead of duplicating "does it exist" checks everywhere, do it in
File_redirection::where. You either get what exists or something to save.
An unsaved File_redirection may be paired with an unsaved File.
You will want to save the File first (using ->saveFile()) and put the
id in File_redirection#file_id before saving.
