PCem

I would find it handy to have a dedicated file extension for PCem configuration files, so that one can run/configure a machine directly by double clicking/right clicking/etc on the configuration file, without having to use the command line.

And to keep things simple I think the .pcem file extension would do just fine as well as being self explanatory for non PCem users.
Of course, the compatibility with the .txt extension could (should) be maintained, it would be a matter of choosing the file extension when a new machine is created.

In addition, I also think that all configuration files, no matter what extension they are saved with, should include a header specifying that they are PCem configuration files and whether they are the configuration of the emulator itself or a machine configuration or any other configuration that may be used in the future, perhaps for plugins or the like. I guess that would also help on operating systems that don't rely on the file extension to identify the file type.

Something like the Bochs emulator does with the .bxrc file extension: