When users install SketchUp on their computer, the SketchUp installer puts an incorrect value in the registry. To solve the issue, follow the instructions below (recommended from Trimble SketchUp support forum).

Click on the link below to run the file. 

Windows

If you have a windows machine newer then Windows 98, use: SketchupFile.reg. When you click the link, you'll see a sample text but you can download the script by clicking the "download" link on the top of the viewer. 

FYI, the .reg file is running the following code script:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\.skp]"
Content Type"="application/octet-stream"

Windows 98 or lower

If you have a windows machine, Windows 98 or older, use: SketchupFile Win98orNT.reg. When you click the link, you'll see a sample text but you can download the script by clicking the "download" link on the top of the viewer. 

FYI, the .reg file is running the following code script:

REGEDIT4

[HKEY_CLASSES_ROOT\.skp]
"Content Type"="application/octet-stream"

Helpful Links

1. Sketchup Pro developer community forum post about invalid MIME type mappings (https://forums.sketchup.com/t/invalid-mime-type/35193)
2. Link to how to fix the registry (https://itslearning.zendesk.com/hc/en-us/articles/202343871-Problem-with-uploading-Sketchup-skp-files)