We recently introduced a new URL for accessing the contents of a file

attachment, ensuring that cached page responses don't reference URLs

that can expire (such as those from S3). This was done for file

attachments, but not for their thumbnails, which were susceptible to the

same problem.
We first noticed a symptom of this issue when moving

https://reviews.reviewboard.org to expiring S3 URLs. Whenever we saved a

caption, we'd see the thumbnail reload. This was happening because we'd

notice the new URL with the new expiration in the metadata and

re-generate the thumbnail information.
Now, the download URL can accept a ?thumbnail=1&width=... query

parameter for accessing the thumbnail image for the attachment.  This

requires that the associated MimetypeHandler provides image thumbnail

generation support (which ImageMimetype does).
There's a performance advantage here as well. Now, thumbnails are only

generated on demand, when accessing the appropriate download URL. This

means less work when first processing a file attachment, and potentially

fewer things to delete when cleaning up.
Since that's faster, we can now reference a @3x thumbnail for images

without taking a performance penalty.

Unit tests passed.
Verified that thumbnails were continuing to work, and that editing

captions was no longer causing a reload.
Verified that thumbnail image files were only being generated for the

DPI I was viewing with.