Error when restoring SQL Server 2008 R2 backup on 2012
Do a Restore Headeronly With File = '...'  If it still fails it's not a valid native SQL Server backup file. It's not an issue of versions. 

Open it with a hex or text editor, what's the file header look like? Maybe it's compressed with a 3rd party solution like LiteSpeed and those require you to use a tool to convert to native format if you don't have it installed in this environment.