Changed iPhone Backup File Type. What to do?
The files in the backup are likely HEIC (or AAE for edits made in Photos). Furthermore, the backup could be encrypted, such that the contents of the backup files cannot be read normally.

You can’t convert a file from one type to another by renaming the extension; if this is what you’ve done, rename it back.
If you tried to convert the file using some conversion process, it may have corrupted the file. If you overwrite the original, it’s gone.