The SketchUp Installer puts the wrong value in the registry when users install the Google SketchUp tool on their computer.
The following steps have solved the problem for other users:
1. Click the Start menu, type in Regedit.exe and press Enter.
2. Expand the menu HKEY_CLASSES_ROOT, and scroll down until you find .skp.
3. Click on .skp, and then double-click on content type.
4. Change from the value SKP to the value application/octet-stream.
5. Restart the computer, and try to upload a .skp file.