I downloaded a Free Wheat.fbx file and imported the content into blender.
I have toggled the Automatically Pack Resources on.
I try to save the file as a blend file, but it gives a warning that an image is missing (I believe it's an Image that was used as texture for the wheat)
Report Error: "Unable to pack file, source path......"
But the folder/path indicated (as missing) is not available in the downloaded folder.
My guess is that whom ever had made the Wheat.fbx file did not include the texture image.