@Holger Buerger
1.There is no option to make apps enabled for a user or an organizational unit in the SharePoint. In the SharePoint, you could set unique permission to uses for list items, files, list, library, page, web part, subsite, site collection.
Customize permissions for a SharePoint list or library
2.Go to central administration -> Application Management -> Manage web applications -> Select the web application -> Blocked file types -> Add the type you want.
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