AphrontFileResponse: avoid alien usages of setDownload()
Summary:
I noticed that - historically - setDownload() could also be used with
false, true, or other weird values to activate a download filename.
So, this change continues to give full compatibility to PHP 8.1 but
with extra validations.
This also adds a bit of inline documentation to put this more explicit.
For more context see the previous version:
rP5952b0a31b6a: Stop mangling filenames when downloading them
rP96ae4ba13acb: PHP 8.1: fixes for strlen() not accepting NULL anymore, part 2
Ref T15190
Test Plan:
- Drop a file in a comment, click, download
- See that it still works
Reviewers: O1 Blessed Committers, Cigaryno, avivey
Reviewed By: O1 Blessed Committers, Cigaryno, avivey
Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno
Maniphest Tasks: T15190
Differential Revision: https://we.phorge.it/D25113