> It is an index-file. Thats no question. But why does
> he changes the link /subfolder/ to
> subfolder/index.html 
> He knows that theres an index file in it...

on your machine will be basePath\\ProjectName\\subfolder
subfolder is a DIRECTORY.
On the server /subfolder/ is an index FILE
A mirror is NOT a copy.
The file must have a name on your machine.