The DS212j supports the synology video station package (I'm sure they all do). Synology gives away a video player app (DS Video) for both iPad and iPhone (perhaps others) that works well. So you load your videos on the NAS, have the video station app index them, view with the iPad and airplay to the apple TV. Many here use it this way, there may be better more elegant ways, but video station and DS Video works well enough. As with all video servers, the hard part is categorizing and organizing you videos (I have about 200 clips and episodes), but you just select each video and categorize it.. and done.
I believe the DS212j media server and/or video station may do some transcode, but all my videos are QT or iTunes compatible, so I dunno and can't offer comment.