The DV input is only for connecting cameras to make DVDs of camera recordings. None of the Magnavox units support playing MP4, AVI, DiVX or WMV videos on thumb drives or external HDDs. They are strictly intended as "timeshift" PVRs for watch-and-erase TV recording, or making DVDs of that material, or playing DVDs. For those tasks they offer excellent performance at a low price.
Five years ago, for double the Magnavox price, there were a few other brand DVD/HDD recorders that could either play files on attached thumb drives or copy them to their HDD for "video jukebox" playback. But PC files tend to be subtly damaged or out of precise spec, so many of them end up being unplayable by these recorders anyway, and the feature was seldom workable.
For whatever reason, recorders are terrible at playing downloaded files, even if equipped with the capability. If you have a lot of such files you want to play on your TV, buy a separate cheap dedicated DVD player with USB connection: these are much more compatible with a wide variety of files on thumb drives or external HDDs. In the age of BluRay, not many DVD players are left, but brands like Philips (sister to Magnavox) are still popular inexpensive DVD player alternatives with USB video file playback.