I believe that your wedding vows are the most valuable and important part of your wedding, and they're not that hard to make awesome. Here's my tips!
Download the MP4, then open it in Files → Downloads. To add it to Photos, tap Share → Save Video. To keep it in Files or iCloud Drive, use Move.
Download the film and find it in your Downloads folder.
Upload the downloaded file through Dropbox, Google Drive or your preferred app. For Google Photos on iPhone, save it to Photos first, then back it up in Google Photos.
If Save Video is unavailable, try the MP4 and check your free storage. Keep a second copy somewhere safe.