Fix accidental rollback of writeFile() fix

Problem/Motivation

In #3282524: Fix writeFile() returning TRUE even if a file has no size → we fixed a bug where writeFile() was returning TRUE even if a file was empty. In the refactor at 🐛 Add a lock around retrieving upstream files Fixed to add a lock, I used the original code as I filed both issues at the same time. When updating, I missed ensuring that fix was in.

This was missed because we only add a new method. The original writeFile() with the fix is there unchanged, because it's protected and needs to be kept until a 3.0 release.