That error comes from the file parser, not the component manager. It means either it's a file type that QT can't read (mkv, ogm, etc), or it's one it can read but is named incorrectly (an mpeg named "movie.avi" will throw that error for example). You can open the file in HexEdit and memorize the way each of the various file types looks, and when wrongly named files show up you can know for sure what to rename them (or you can try names until it works). It's very easy to tell them apart (for example matroska files start with the 8 characters "matroska", AVI starts with RIFF, mpegs start with about 100 pages of nothing after a 00 00 01 BA, etc).
VLC works too of course, and doesn't balk at wrongly named files. But it's always good to have options.