While this is naive way to add new case there are couple of problems with incrementFileName in general:

1. It's always using . as separator of version prefix/suffix, it would be good to support also other separators (like -, app-1.js should be incremented to app-2.js)
2. Since it's just parsing version string to int we lose leading zeros (right now app.0001.js is incremented to app.2.js which is probably wrong behavior.)

@Krzysztof-Cieslak thanks a lot for your PR.
I would be happy merging this in if we add some unit tests - they would increase my confidence and would use as a documentation how this beahves. Could you export function incrementFileNameand add a fileActions.test.ts and test out that method.

@isidorn, I've added couple of tests. Should I also tackle those 2 problems I've mentioned in an earlier message?

@Krzysztof-Cieslak yeah that would be great! Adding tests for those cases would also be awesome! Thanks

This is ready for review.

@Krzysztof-Cieslak thanks a lot! I love the tests, they give me confidence looking at all these regexes.
Merging in, thanks again!