Can you please elaborate your requirement, as far as I know the option to specify the html file extension string is used to set the file extension of the HTML generated after transformatoion through Dita OTK, but why do you require it and how would you use it as RoboHelp just has extension .htm to all its HTML files created through any method be it import or create new topic.
Added to that if you want to modify any of the DITA OT build arguments apart from what is provided through RH UI, you may use the scripts used in the DITA OT itself.
RoboHelp uses the scripts placed at the location of DITA OT that is pointed to by user in the DITA OT import dialog.
Hope this helps...
I am getting files with a .html extension as a result of the import DITA map feature (I was expecting .htm, as you mentioned). Thankfully, this is a smallish pilot project, so I could change the name of each file manually in the project.
The .htm file extension is a requirement because of the way our help topics are currently mapped. I need to reproduce filenames from DITA source, and these file names are uniformly .htm.
I did try changing the extension in the various xsl scripts for the toolkit's HTML transform, but none of this changed my results. But this is usually passed as a parameter in the build script. I don't see how RoboHelp could be using the build scripts from DITA-OT without modification. One of the parameters you need to pass is the source file name. If you are talking about the XSL scripts, that's a different matter. You can change those all you want, but this particular parameter will always be taken from the ANT build script, and changing that behaviour would require a fairly substantial rewrite to the toolkit XSL code. There must be some facility happening behind the scenes in RH to construct this script.