More code per #357 - handling of duplicate files.
If an uploaded file appears to be a duplicate of an existing file, *by content* (i.e., by md5),
a warning message will be displayed, and the matching md5s highlighted.
This way the user has an option of either canceling the entire upload, or checking the delete
chekboxes next to the files that don't want, before they hit 'save'; or just to proceed with
adding the files as they are - if they have some kind of a weird reason to have multiple identical
files in the dataset...