HTC One SMS Backup File with no Extension Restore issue
Just changing the file extension will not help. As far as I know it's just some kind of encrypted text file. It's either a htc or an android specific file. In order to restore a message from that file you need an android device. It might work with the stock android sms app in an emulator but probably not. I suspect that it's only going to work with htc sms app.

[edit]:
The backup file is an encrypted sqlite database file not just some text file.