When using MPEG mode ZoneMinder can replay events in encoded video format. However what formats are handled by the browser varies greatly between machines. This option allows you to specify a video format using a file extension format, so you would just enter the extension of the file type you would like and the rest is determined from that. The default of 'asf' works well under Windows with Windows Media Player and 'mpg', or 'avi' etc should work under Linux. If you know any more then please let me know! If this option is left blank then live streams will revert to being in motion jpeg format