Fix wrong image file dimensions in "Default Alt Text" on File pages
Summary:
Rendering a `PhutilNumber` as a `%d` cuts off digits after the first comma, while `%s` does not. See https://we.phorge.it/D26546#39386 for gory details.
Closes T16454
Test Plan: Go to http://phorge.localhost/file/, upload an image file which has more than 1000px width and/or height, look at the "Default Alt Text" value in the "File Metadata" section of the corresponding File page.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16454
Differential Revision: https://we.phorge.it/D26683