[v3.x.x] Support attaching directories

[dlqqq]

trafficstars

Problem

The attachments API does not support directories.

Proposed Solution

Determine how directory attachments should be supported by the backend (while respecting the model's token limit), then implement it.

Additional context

This issue will be targeted in the v3.x.x milestone, meaning that we currently do not plan to develop this until after the v3.0.0 major release.