Sailfish OS is capable of playing media files in formats like MP3, MP4 and WMV. But media
containers like AVI are not officially supported since required codecs are missing. "Xerxes2" created a package which installs many additional codecs and makes your Jolla capable of decoding many known media file formats.
Finally codecs are also needed to read converted files: The "Ultimate Codec Supports" for Sailfish OS is a great codec pack which is available via Warehouse application (OpenRepos). It's necessary to enably "sailfish_lib" repository. To do this open Warehouse app and select Browse by categories. There you find a category called Libraries including the needed repository.
Now enjoy your music and videos on Sailfish media player on any other 3rd party application!
Direct RPM download: Ultimate Codec Support on OpenRepos.net