Description: I noticed that if I play files in mplayer using firefox "open with" download option, then ~/.cache/gdm/session.log can grow very large because of the output of mplayer. This though doesn't happen if I open a file from nautilus (I am using gnome). I'm guessing other DEs might have a similar issue.

I can see an older version of the package used to pass the -really-quiet option to mplayer so it generates no output but then it got removed.
https://projects.archlinux.org/svntogit/packages.git/commit/trunk?h=packages/mplayer&id=d73ff6beb888f8b2f45e19cd2a0deb9b0b8ca60e
https://projects.archlinux.org/svntogit/packages.git/commit/trunk?h=packages/mplayer&id=1d3c22b53e829dca79ecc560d625bacef2c67f1a

I think a better solution is to change Terminal=false to Terminal=true in mplayer.desktop so a terminal window opens up every time mplayer is run from its .desktop file so the output is only shown in it. This also makes more sense because otherwise if you play an audio-only file there is no obvious way to stop mplayer (and after all mplayer is a cli application).

Steps to reproduce:
1) wc -l ~/.cache/gdm/session.log
2) open a media file using firefox "open with" and choose MPlayer Media Player
3) let it play for some time (note that mplayer has constant output as long as a file plays)
4) wc -l ~/.cache/gdm/session.log