When you submit a document to the court you must "lay a foundation" for it. That is, someone with personal knowledge of the document must testify under penalty of perjury about the contents being accurate and unmodified, etc.
It takes a certain degree of chutzpah to introduce those forged documents in court, though… you’re submitting them to adversarial scrutiny, it’s part of the court’s job to decide whether they’re reliable or not, and detection leads to criminal sanctions
I mean the other way around - you have to prove someone is guilty beyond reasonable doubt. What if there is some genuine footage, but there’s enough doubt that it could not be genuine