How to ask for an extension on a manuscript review? I accepted a request to peer review a manuscript from a journal, but was not able to view the attached files correctly and reached out to the journal three times about this problem. They finally responded and addressed the issue, but now I am less than 3 days away from their original deadline for submitting the review and I know I need at least 1-2 weeks to do an adequate job. How should I ask or state my request for more time? Is it necessary to provide the reason or justify my request for the extension in my email?
For added context, this is a journal that I'm likely to submit to in the future, so I would like to maintain an amicable relationship with the current editor.
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.