[Plugin: WP Smush.it] Smush.it not working on IIS

Smush.it leaves all sorts of .tmp files int the wp-content folder when installed on an Windows IIS server. Then, it doesn’t copy the smushed .tmp files over the original files.

Is this a permissions issue? (I kind of doubt it.)
or
Is it more that smush.it is hard-coded with unix paths?

thanks,

https://www.ads-software.com/extend/plugins/wp-smushit/