Resolve a fetch-able URL (e.g. presigned S3 URL) for any file entry.
Longest edge of the resulting thumbnail, in pixels.
OptionalsignalAborted when the requesting card unmounts or its file changes (e.g. a
filter switch). Generators should pass it to fetch and bail out early
when it fires so stale work doesn't delay newly visible thumbnails.
The file needing a thumbnail.