> All the files ends with a .HTM extension but once 
> downloaded they are all renamed with .html extension.

This is in fact a minor bug ; as httrack does not 
rename .htm files (but it does with .HTM - case problem)
There is no way to bypass it, however

I will fix it for the next release (the bug is fixed with 
several other ones - I will have to rebuild an intermediate 
testing release soon)