Every local file you receive from the Internet have a content type. It is used to help internet application to open files with an appropriate method (HTML pages, ASX metafiles, SWF flash objects,... are opened differently).

The flash .flv video on this web-site uses "text/html" content/type, the same one that is used for HTML web-pages. Replay Media Catcher filters HTML video and audio streams by content/type. The content itself is not analyzed yet. It might be in the future, especially if such way of broadcasting flash videos become more widespread.