Setting mime type - doesn't seem to work a the site level

I've been working on setting mime types with powershell.  When I set them at the site level, they don't seem to work.  When I set them at the top (?) level [the server name node in the IIS Manager], they work.

I did this with both powershell scripts and the IIS manager.  Behavior seems to be the same.

Is there some magic here?