Enclosure wasn't checking if file existed on disk. Hence, if the
enclosure was reporting a file size of 0 (which matches the non-existent
file size, then it would set the status incorrectly to Enclosure::Downloaded.
The file size in the database will be corrected if it doesn't match the
real file size.
The "new" status will be unset once the file has been downloaded.
Images are now stored in the cache directory in a dedicated subdir
called "images".
Enclosures are stored in the data directory in a dedicated subdir
"enclosures".