Consider allowing size-time tradeoff for uploaded media.

[Biep]

Allow size-time tradeoff for files Matrix is very useful to transfer files, but most sysadmins don't want to set a high size limit for uploads, to avoid excessive storage requirements. Both concerns could be served if the sysadmin could set a function from size to retention time (in the most basic case a simple t=f(s)=constant/s). This would mesh well with the request for self-destroying messages. Messages might be kept longer on some servers than on others, and in that case they could still be called up for download.

[erikjohnston]

We'd probably want some way to allow clients to specify whether they are OK with the trade off. Being allowed to upload a 1GB file for day is great, but only if you know that it will disappear tomorrow. I'm not sure how that would work, but probably requires changes to the spec.

This is probably something we'd consider accepting a PR for, if the above is figured out.