Two Issues: File system config / filemanager mod & "file is not a valid acidfree elment type (application/octet-stream)

What up,

Converted Joomla CMS user here...To make it quick, I'm having two issues:

1) Can only view image pics when I set administer >> File system configuration to private (public not working)...Could be that I'm a newbie, and I'm not getting something, but I've set file path to www.domain.com/drupalfolder/files, as well as chmod to world readable/writable

2) Can't upload .flv or video types using "Mass import" functionality (getting following error message: "file is not a valid acidfree elment type (application/octet-stream)"...I've done everything documentation has mentioned: installed views; image; filemanager (all latest Drupal 5 -V's)...Noticed something about needing mplayer in doc, but was unclear...Is this a necessary install on client browser uploading video file or those server need some package installed?...

Much Appreciated,
Ian