No, saving a file from an e-mail wont help, nor can you make any app a default. If any of your video players were able to open that file, they would have shown up in the "Open In.." box in the e-mail.
You'll need to try another app to see if it'll open the file. My suggestion would be OPlayer HD, which you can get free in the App Store (there's also a paid version ... But try the free one first to see if it'll work). Another app to try would be Goodreader, which is $4.99, USD, in the app store. But, again, it's paid - and a bit much for a "try and see if it works," although it's a great app.
So, try the OPlayerHD and use it in the Mail app. If its going to work, it'll be an option when you press and hold the file.
Good luck.
Marilyn