@aleksei Sounds like potentially a cool idea. But also potential for abuse. For e.g. many forum admins invest significant effort to foil file scrapers, etc. and this could facilitate/exacerbate. Maybe mitigated to some degree by permission restrictions to logged in users but..... then the bots just create a sock puppet account.
Another potential issue: long running threads sporting hundreds of images, etc. How to display? Could be a UX conundrum, eh?
Hmmmm... Still could be interesting for some situations.
Not a dev but will hazard a guess that this might should be a plugin rather than core front end function as you suggest. Sorry I cannot offer more targeted insights.
In general, we recommend MongoDB if you have a large community. Redis will work just fine, although the memory usage does scale up over time as everything is stored in-memory. Just something to consider.