Ah, I think I’ve missed the point, or, in any event, the characters in the file names are not the problem. I submitted a file called το ελληνικό αλφάβητο είναι το καλύτερο-3_1_9.pdf and was able to download it through the Enketo edit screen without any issue.
@Stavripal, if you cannot retrieve these files by going through the steps I mentioned above (clicking the open 
icon or downloading a ZIP export), then they do not exist on our server.
When submissions contain attached files, they can be uploaded by Enketo or the Collect Android application in multiple parts. This makes it possible for the KoBo server to receive incomplete submissions that contain the text responses and some—but not all—attached files. An example of this is shown in @Kal_Lam’s screenshot:
However, the data collection client (Enketo or Collect) should never delete these submissions until they have been completely uploaded to the KoBo server with all their attachments. The next step here would be to check Enketo or Collect, whichever you use, on the devices used for data collection and verify that no submissions are still there waiting to be uploaded.
