It looks like this is now possible via the properties, but what’s inserted is "![[attachment.ext]]" which doesn’t work until the exclamation mark is removed as previously stated. However, dragging the same attachment into the source front matter inserts ![[attachment.ext]] (no quotes). In both cases, the former seems to make sense to be inserted and for Obsidian to then support the "![[]]" style.
My use case is this:
---
related:
- "[[note]]"
- "![[attachment.ext]]"
- https://obsidian.md/
- file:///C%5Cfile.ext
---
It could also be nice to have an option which toggles whether linked notes/attachments via front matter / properties appear in the local graph or not, as depending on how they’re used, they may add unwanted clutter to graph navigation.