This suggests that you are trying to install a file that is not actually the plugin. How do you try to install the plugin? Where did you obtain the plugin jar file?
If the problem persists, try to determine what file type the downloaded file is. Maybe look at the file size, or try to open the file as a text document. Look for any clues to find out if the plugin file actually is a plugin file (and not some kind of error message, or otherwise corrupted file).