Are you sure that the wallet file doesn't have the extension? Or it might be just hidden? check carefully because there are some users here who mention about 80MB of the wallet.dat file of their Bitcoin core wallet.

Here's the post below for reference
- https://bitcointalk.org/index.php?topic=34028.msg2570257#msg2570257

About the 12 phrase seed can you try to check the words from your backup seed with this list of words from here https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt
Check if all of the words from your backup seed are under that list. If some of the words are not there it means it's a different wallet.

It might be a litecoin or other coin. Can you try to check if when it was created? Try to right-click the file that has 12 phrase backup and click properties it should be show details with the date when it was created.