File '<file>' cannot be shared in global assembly cache folder; only assemblies are allowed
You have placed a file that is not an assembly in the Global Assembly Cache folder. The global assembly cache can only contain strong-named assemblies.
To correct this error
Double-click the error message in the Task List to go to the file in the File System Editor.
Move the file to a different folder by using a drag-and-drop operation or the Cut and Paste commands.