Fix "original format" downloads -- missing DIRECTORY_SEPARATOR

This patch adds a DIRECTORY_SEPARATOR between the file's path and filename when using the download function.  Without this patch, "Original Format" downloads do not work -- an empty response is returned, with no download. 

The patch also changes the system's behavior to return a 404 if the file is not found, instead of returning an empty response with no download.  Hopefully the DIRECTORY_SEPARATOR patch will ensure that this does not happen in regular operation, but if the file is somehow missing I think a 404 is a better signal that something is wrong.