Replace 'original' file with 'download' option

[samwilson]

Currently, a full original image file can be downloaded from e.g. /P433F.jpg, which includes all original metadata. It'd be better to make this serve a modified version of the original, with the metadata replaced by that from the Post. This would mean that downloaded images could have a link back to the post's URL, and correct author and location information.

Site admins would still be able to download the original.

Probably, a new letter code should be used, e.g. /P433O.jpg (O for Original; F for Full).