Supported image file types for "Preview" in OS X:
BMP, JPG, GIF, PNG, PSD, TIF (there are others, but those are most common)
Supported video types (QuickTime-natively):
MOV, MPG (Most), MP4 (Most).
Supported video files types (VLC Media Player-http://www.videolan.org/):
MOV, MPG, MP4, AVI, FLV, WMV, MKV