Montage File List URL image load interface is broken

[geertivp]

When loading images into a new campaign using an URL the load finishes with a (fatal) error message: Import Warning/ import issues - no entries imported - all entries disqualified by round settings

No images are loaded, the campaign stays empty...

See https://phabricator.wikimedia.org/T279386

Currently we can only load the image list using 1 single category, or via copy/paste of a verbatim list of file names.