In Sitefinity there are predefined icons for certain file types. However for some extensions like .xlsm there is no icon defined in our assemblies.
However you can create your custom icon to be displayed for the .xlsm documents as shown in this short video
The image file used in the video is attached to this message for your reference.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items