.swf files that contain a period in the filename can not be imported.
Rename the file and replace the period (".") with an acceptable character like an underscore ("_").
For example: If the file was named interstellar.voyager.swf it could not be imported because of the the period inserted between interstellar and voyager, but if the file was renamed to interstellar_voyager.swf it could be imported successfully.
Mac OS X (all)