Sorry, no way to rename them. Only one or two times in the 20+ years of using RoboHelp have I ever seen someone post about absolutely needing .HTML file extension.
You can certainly advise the development team about the need. It won't help now, but might factor into a future upgrade.
Our end user has a filter that prevents them from being able to open an HTM file. 5 groups that can access the WEBHELP, and one that has a filter which is preventing their ability to view the files with HTM extensions. Any suggestions for a work around?
Yes, but you probably won't like it.
Remove the filter?
Ducking for cover...
Seriously? A filter on .htm? What's the rationale behind that? I can only concur with Rick suggestion: remove the filter.