Download count: Invalidate file cache tags only when needed

Problem/Motivation

In download count module we implement hook_ENTITY_TYPE_access for files which adds a file download count for every downloaded file. However this code is also triggered for comment attachments. Depending on the settings or user permissions some files are excluded and the download count won't be adjusted. The file cache tags are invalidated for every file download.

Proposed resolution

Let's do the invalidating of the cache tags only when the file download count is actually changed.