When trying to drag and drop a .jpg file into the zone for uploading, I get a pop-up dialog box that says "FileExtensionNotAllowed". The "Processing..." animation then continues until I cancel.

If I click the "...or browse file" button and then select that same .jpg file, the upload fails and I get the inline message "⚠️ Error: Only .gbx and .zip filetypes are allowed."

EDIT: I figured out that I could only add screenshots for an item set after the set is uploaded.