Feature request: Drag and drop PDFs, Word and Excel documents into agent conversations

Problem

When I work with coding agents, my inputs are not limited to source code. I frequently need to hand an agent reference documents (PDF, Word, Excel), images, and other local files. Attaching these files should feel as straightforward as dropping them into a chat. Today, some non-text attachments fail before a message is sent, requiring manual workarounds.

Desired outcome

Drag local files directly onto an agent conversation or its composer, attach them reliably, and let the selected agent use the content or file path as supported. No requirement to first open a file in a code editor or manually paste a path.

User story

As someone working mostly through AI agents rather than code editing, I want to drag project documents directly from Finder into a conversation so I can give agents the same context I would share with a colleague.

Acceptance criteria

• Drag-and-drop supports at least PDF (.pdf), Word (.docx), Excel (.xlsx), common image formats, and text/code files.
• Files can be dropped onto the conversation or message composer without opening an editor tab or moving them manually into the repository first.
• Selected files appear as identifiable attachments (name/type, removable before sending); multiple files can be attached in one action.
• The receiving agent gets usable access according to its capabilities (e.g., an accessible file path for a local agent, or extracted/encoded content when the protocol/provider supports it).
• No silent failures, unexpected composer clearing, or generic “Binary files are not supported” errors when a safe, compatible handoff is possible.
• If an agent/provider cannot handle a file type, explain the limitation before sending and offer an actionable fallback such as sharing the accessible path where applicable.
• Large files and unsupported formats have explicit, helpful feedback; avoid embedding unnecessary binary data in prompt text.

Design principles

Fast, predictable and minimal. Files should be first-class conversation inputs, not an editor-specific feature. Keep the UI lightweight; don't force users through modal import flows.

Related existing bug

#56590 – Attaching non-image binary files to agent message editor fails with “Binary files are not supported” reproduces PDF/XLSX rejection in an ACP thread. This request covers the broader interaction and expected behavior across file types and providers rather than re-reporting just that bug.

Related agent-first request

#66203 – Agent-first workspace with an optional editor.