Custom .omt extension for zip bundle

### Is your feature request related to a problem? Please describe
I use packages which are normal zip bundles but with a different extension (.omt).

### Describe the solution you'd like
I would like Keka to recognise my .omt files as something that can be handled just as .zip files (for extraction)
Being able to pack the contents of a folder with my custom extension would be great too!

### Describe alternatives you've considered
If I rename the file (changing .omt to .zip) I can extract the bundle, but I would like the tool to recognise my custom extension.

### Additional context
These .omt packages are OmegaT projects, packaged so that it's easy to transfer the file.