I go the following error message for one of my uploads in the Google Photos app on Android Lollipop (5.1.1) on Google Photos version 18.104.22.168391081:
Not backed up. Too big or unknown type.
With the only action ‘Got it’. Why did I receive this error message? I did not hit my Google storage limit. And the video was 1080p, so within the limits. The filetype is a simple .mp4 video file, so that would be no problem either.
The issue ultimately was the filesize, which is (probably) limited to a certain size. I could find no evidence of what the file size limit is, however. The video file that I tried to upload using the Google Photos app on Android was about 3,5GB.
Did you find more information on this? Let me know below by leaving a comment. Thanks!