"We know that this link includes material that is illegal to possess, and it is on your server."
"We don't know the contents of the files on our server, so we can't know that is was illegal"
"Fine, delete that file, and we won't charge you for possession this time. Now that you know your service is used for this illegal material, you need to stop hosting material like that."
"How, if we don't know what's in the file sent to our server?"
"... maybe don't take random files you don't know about, and share them on the open web with anonymous users?"
That’s not how csam reporting works at all. You aren’t punished for csam being on your server, as long as you do something about it. You can easily set up cloudflare to block and report csam to the NCMEC (not the FBI) and it will all be handled automatically.
"We don't know the contents of the files on our server, so we can't know that is was illegal"
"Fine, delete that file, and we won't charge you for possession this time. Now that you know your service is used for this illegal material, you need to stop hosting material like that."
"How, if we don't know what's in the file sent to our server?"
"... maybe don't take random files you don't know about, and share them on the open web with anonymous users?"