If the user uploads a .pot file to File project, translates it, then downloads it, Zanata serves a file with no extension and displays ".null" in the menu option.
1. Create/access a File project and version
2. Upload a .pot file for translation
3. Select a language in the languages tab, then click the V (down) arrow on the file
Note the option indicates "Download translated [.null]"
4. Select the Download option
File downloaded without extension
File with .pot extension downloaded
I think maybe the bug here is that it allows a .pot upload to a File type project...