Fix: don't override files with the same name

[xjcb-de]

I run into a case where there were multiple files in the HAR that had the same filepath and filename. The ZIP would then only include one of them.

This PR alters the filename if there's already one existing and adds a numbered suffix:
file.png --> file_2.png --> file_3.png ...

[JC3]

Thanks. Will this work if the filename has no extension?