The last tricky part was writing a SQL migration to populate the sizes of files that were uploaded and stored in the DB before this change. I've never written an update that derives from other data in the DB before, but it wasn't too hard.
Hope you enjoyed following along! Here's the full PR with the fix: https://github.com/mtlynch/picoshare/pull/221
Update! @wmertens and @dholth showed me a simpler way to achieve the same performance without storing the file size redundantly. Creating an index achieves the same thing.
Creating the index is a one-line fix, and it doesn't have the drawback my original PR did of storing a redundant copy of the file size. https://github.com/mtlynch/picoshare/pull/230/files