Proposal: Keep normal image attachments on the thumbnail shelf

[eliophan]

Problem

Attaching an image can show it twice in the composer: as a thumbnail and as an inline image chip. This makes the composer look cluttered.

Proposed behavior

Keep images on the thumbnail shelf without automatically adding an inline chip for normal picker, paste, or drop flows. Add a chip when the user selects text to replace, or when the editor cannot accept input. Keep file attachments as inline chips.

Would maintainers consider this the intended behavior? Please confirm the expected scope before I resubmit the change.

Context: [PR #18363](#18363)