How to secure your servers from content uploaded by users?

Question

Information Security

The user can upload mp4, mp3, svg, giv, png, jpeg, webp and possibly in the future Browser HTML / JS + Mobile games, he hasn't decided on the exact file formats.
How to secure your servers from the content they upload?
How do sites like YouTube, FB, VK, Instagram and others do it?

Answer 1

Usually, social networks and large platforms re-encode downloaded media files, in this way they get rid of metadata and specially formed byte sequences that can be misinterpreted by vulnerable programs.
Against content - it won’t help if illegal content is uploaded there - you still have to answer to the law, there are no ways to determine prohibited information (because each country has its own laws, somewhere even a harmless picture with Winnie the Pooh is imprisoned) .

Answer 2

The question "How to secure" should be accompanied by a model of threats against which it is necessary to protect. What exactly are you afraid of - attacks on the server, attacks on clients (this is if one person uploaded a malicious file, while others download it and have troubles), rights holders' claims?

Answer 3

How to secure your servers from the content they upload?

From what to protect something? From a site breach? Or from uploading a loli video? If your site can be attacked with user-generated content, then obviously the site itself needs to be improved.
And if you protect against illegal content - only post-moderation. View and delete. And prompt response to complaints. Do you think why the pipe of moderators is gaining in batches? Because watching all sorts of crap and abomination (CPU, guro, stupid jokes, etc.) is very difficult. It would seem - well, what is it? But try it :)