By replacing action download by downloadrev,
XWIKI-15450 is introducing a regression on SEO. Action download is allowing the distinction between current version of attachment vs historical versions served using downloadrev. This is (could be) used in robots.txt for example to filter robots crawling on historical version.
There is no need to change the download action into downloadrev action to solve the cache issue encountered with
XWIKI-15450. Simply adding the ?rev= parameter to the generated download URL is sufficient, and this parameter does not need to be considered by the download action. It will only be there in order to help caches manage a correct cache key.