typefinding: fails to detect certain SubViewer 1.0 subtitle file

Submitted by Bastien Nocera `@hadess`

Created attachment 162022
misdetected subtitle

gstreamer-plugins-base-0.10.29-1.fc13.x86_64

The attached file is detected as text/plain when it's actually a subtitle file.

$ gst-typefind beowulf.sub
beowulf.sub - text/plain

Attachment 162022, "misdetected subtitle":
beowulf.sub