How can I upload videos to Derpibooru?

Currently, I wanted to upload this video – https://www.facebook.com/mylittlepony/videos/2278297618854329/ – to this website, but even when I tried to convert it to Webm, it does not work and instead displays “HTTP Error 500”. The video is about EqG Rainbow Dash kicking a ball, and it is an official animation by the MLP franchise’s makers.
 
Would you like to share some help so I can upload videos to this site as others did?

@HorsesandMuchMOAR  
By convert, I take it you mean you used something like VLC to do a proper file conversion, right?
 
Is the resulting file size still below 25mb?

@The Smiling Pony
 
No. I just copied the link to an online converter, and it is 1.49 MB with a long gibberish name from being generated by the website. Can you help me learn to use VLC to convert the Facebook video?
 
In another try, I converted it to MP4 (again with another online converter), and it is 899 KB.

@HorsesandMuchMOAR  
If you have VLC, you can open it up and hit Convert/Save in the File menu. Can add a local video there, or something online if you have a direct link to the file. Once you have that and hit Convert, can chose to save as webm.
 
That should work.

Sounds like the problem is more that that direct facebook link probably has some kind of hotlink protection. If you’re not too scared of the command line, just use youtube-dl and then convert it locally in your favorite whatever.  
^{No, I’m not going to explain how to use youtube-dl. It has a pretty good manual.}