This simple hack will open MP gbx'ses in MP, and TMF ones in TMF . It also works with replays.

1.
Put the following code in a new .cmd file.

... says:

ECHO OFF
@find /c /i "exever=""3." %1 > NUL
if %ERRORLEVEL% EQU 0 (
start C:\Progra~2\ManiaPlanet\ManiaPlanet.exe /useexedir /singleinst /file=%1
) else (
start C:\Progra~2\TmUnitedForever\TmForever.exe /useexedir /singleinst /file=%1
)

Save the file as .cmd, not as .cmd.txt or something weird like that. In the generic Windows Notepad you should select Save As -> 'All files' in the drop-down box.

Edit the two bold sentences to the actual path to ManiaPlanet and TMF. Progra~2 means the Program Files (x86) folder.

2.
Select any challenge/replay (either TMF or MP), right click it and select [Open With]. Select your just made .cmd file there, and tick the checkbox at the bottom to always open it via this .cmd file.

3.
OPTIONAL: Restoring the GBX file icon.
Press [WINDOWS] + [R], type regedit, hit enter.
At the LEFT side, browse to: HKEY_CLASSES_ROOT -> Applications -> [The filename of your .cmd] (in my case RunGame.cmd).
Right click this filename.cmd (still at the left pane) and select New -> Key. Call it DefaultIcon.
Left click the just made DefaultIcon key. At the right pane you should see a (Default) called value. Double-click it to edit the (Default) value. Set this value to the icon's location, in my case:
C:\Program Files (x86)\ManiaPlanet\gbx.ico
In the end it should look like this:

Sign off your computer, or just totally restart it. The icon should be there now.

I hope this little hack helps some of you :3.

PROGRA~1 means Program Files folder, and PROGRA~2 means Program Files (x86) folder

Is it work for replay dl to, ?
i mean if i dl a replay at tmx and opens it, will it start tmu ?
and if i open a replay here at mx will it open with mp. ?
or is it just maps. ?

just maps. replays both in TMUF and TM² are called like blabla.Replay.Gbx, so it's impossible to know if it's from tm1 or tm2.

If you download a replay it'll open in TMF. You can change it so it opens in MP by default tho.

EDIT: Adapted the script, it'll now work with replays and everything else as well.

you're checking the version of program, with which the file was made?

Yea, it'll check if the version is 3.0.0 (= maniaplanet). This works with every gbx file so it's pretty much flawless.

until they update the ManiaPlanet exe

-edit- this seems to get rid of the .gbx icon ... does anyone know where to find the gbx icon so it can be changed back?

Bucky says:
until they update the ManiaPlanet exe

Till now they didn't update the exe version yet, it always was 3.0.0.
I'll check for the icon, sec..
Added a guide for restoring the icon.

wow, very help full little program thx alot.

If you install the game elsewhere just change path in cmd file.