I unpacked the zip, then add an extension to the file.
A zip is an archive file (1) even if some idiots use zip as file(s) in their application(s).
(1) eventually, it is a way to share many files in one archive file (container)…
And… Tim is 100% correct…
No, it is a bundle. On macOS, a bundle is a folder with special contents and application specific data. The Finder shows it with the defined icon as a file; a convenient way to shade many files as if they are one (you never forget to share a file).
This is the explanation.
Right-Click on the .prproj “file” and choose to show what’s inside (the fourth entry in the Contextual Menu something like “Show the file contents”…).
Right-Click = Ctrl-Click (left click)
Here’s a screen shot of your archive file as is (it was downloaded that way) and the resulting uncompressed file with an xml extension:
Regards