I can see that this is doing an A380 Airbus on you.
"QT plays ALL EXPECTED TYPES" in other words, the known types IT CAN play. This does not mean to say that your default player will not load up a file because it does not know the format, even basing a file on its extension type is not an indication of a files playability as you have the issue of Codecs to consider and the file could not be a media file as file extensions are not unique to a particular product or service but the player will attempt to play a file of a "Known" extension type assuming that it is a media type.
Did you spot anything? in particular... like in the notes? like in the table towards the bottom? like "mpegfile | Movie Clip | QuickTime Player Application | QuickTimePlayer.exe"
If your having issues with playing MPG in QT then the issue is with your version of QT or lack of codec and I would go with the latter.
So .flv, everyone is doing it, would you jump off a bridge just because others were doing it?
All I can say is that I am pointing out issues that you should consider when choosing a media file type and who your looking to reach, if all your users use Flash then fine, that quite dandy but your missing the issues of people who may want to view but do not want to install Flash or are using a browser type that just does not support flash (yes they do exist and are not a figment of my advance imagination and K step thinking skills.).
Mpeg has been around from year dot, Flash has not and the video aspect is very very shiny and new, so new you can still see ya face in the chrome work.
Like any video codec, you need to know how to use it and just because .flv appears to produce "Better" q factor than other container formats to me is a pretty dumb argument to take up, theirs plenty of existing video formats that out perform Flash Video.
So why do you have issue with just offering up an alternate format for those who do not use flash? All it would take is a little time, putting a link in and allowing people to choose, why do developers have to be so "Myopic & Blinkered" on such subject? Some of the best sites out on the internet offer up multiple formats for people as they seem to recognize that this is a world of choice and not one where an individuals ideal is enforced on the masses.
For some reason people seem to think I pick fault with them, I do not, I may not be subtle or as tactful as some people here but I believe in telling it like I see it in plain English that seems to constantly do an A380AB on people. I am truing to kick you in to the reality of the situation and that is one where you seem to be enforcing your ideal on the masses rather than catering for them.