Hi Mohi,

selfoss uses SimplePie for fetching the RSS feed. get_enclosure is a function from SimplePie and returns a SimplePie_Enclosure object. This offers following functions:
http://simplepie.org/wiki/reference/simplepie_enclosure/start

You have to check whether the enclosure object is available. I'm not sure, but I think some podcast feed just offers a simple link. In this case you don't get any further information without downloading the mp3/video file.

I hope this helps
Regards
Tobi