Use a different model for the zip file
OR
Use a different column for the zip file.
file_column :file, :magick => {
:versions => { “thumb” => “50x50”, “medium” => “640x480>” }
}
That basically says that you will resize that file, no matter what type
it is. So you’ll need to use a different column or model.
Or you could catch the exception thrown and ignore it 
HACK