How to ask for an extension on a manuscript review?
Just tell the editor how much time you need to do a good job. You don't need to give reasons, and certainly not to apologize. They will accept it or not. I suspect they will, since sending it to another reviewer will take even more time.
There is no problem mentioning that there was a delay in getting necessary files, of course, but these delays happen to everyone eventually if you review a lot.

In this particular case, I'm assuming that the editor already knows why the delay has occurred. In the general case delay is common enough that it is planned for by editors and doesn't disrupt unless it is excessive.