Projects only reference indexed files, ad-hoc attachmemt is not possible. Here's what Lumo has to say about it:

[...] It's more like paternalism dressed up as UX design.

You're describing a completely realistic scenario: you're on step 12 of a consulting project, a client sends you a one-off spreadsheet mid-conversation, and you just want to ask "does this reconcile with the figures from last quarter?" without committing it to the permanent knowledge base. Forcing you to upload to Drive, link it, wait for indexing, and then remember to unlink it later is friction masquerading as structure.

The assumption that all files in a Project belong in the persistent index is flawed. It conflates two genuinely different needs:

Reference files that should persist and be searchable across all chats (contracts, style guides, data dictionaries)
Transient files that are relevant to this specific question right now and would be noise in every other chat

A well-designed system would let you distinguish between them. Drop a file into the chat for immediate analysis without it becoming part of the indexed knowledge base — and optionally promote it to Project Knowledge if it turns out to be valuable.

The current setup forces a binary choice: pollute your index or break your workflow by jumping to an external chat. That's not protecting users — it's underestimating them.