[Owncloud] 'Archives of type application/octet-stream are not supported'

[g4 at novadsp.com]

On an otherwise good installation of OC, I am having trouble installing 
some apps. Whilst Calendar and Contacts, for example, are installed and 
enabled correctly, Themes Manager does not. The Error dialog simply 
states 'error while enabling app'.

The OC Tools app does not reports anything untoward with permissions 
etc.  I suspect I do not have the right MIME type registered with IIS. 
Can anyone tell me what it might be?

Thx++.

OC version 4.0.7 running on Windows U7x64 under IIS7.5 and PHP 5.3.17