It's a dif file. It allows you to see the new mount on your pserver.
You have to have game rev 2089, 2019, or 1765.
Save this dif on your HDD in txt file and change file extension to *.dif
(for example: newhorse.dif)
Use dif patcher:
And there select your game file, newhorse.dif and push "Go!"
Now you can see new mounts on your server

Of course if you have new mounts