Desktop app: support drag-and-drop / attachment of PDF, DOCX, and Markdown files (parity with CLI)

Summary

The Claude Code CLI supports drag-and-drop of arbitrary file types — including .pdf, .docx, .md, and other document formats — directly into the prompt, and they are read/processed as input.

The Claude Code desktop app does not appear to support attaching these same file types. Only a limited set (e.g. images) seems to be accepted, and dragging in a PDF / DOCX / Markdown file either does nothing or is rejected.

This creates an inconsistent experience between the two surfaces and forces desktop users to fall back to the CLI for any workflow involving document input.

Steps to reproduce

1. Open the Claude Code desktop app.
2. Try to drag a .pdf, .docx, or .md file into the prompt area (or use any \"attach file\" affordance if present).
3. Observe that the file is not accepted / not attached.
4. In the CLI (claude), drag the same file into the terminal prompt — it is accepted and its contents are made available to the model.

Expected behavior

The desktop app should support attaching / drag-and-drop of the same file types the CLI supports, at minimum:

• .pdf
• .docx (and ideally .doc, .odt)
• .md / .markdown
• .txt and other common plain-text formats
• Ideally: any file type the CLI currently accepts, for full parity

Why this matters

• Workflow parity: users shouldn't have to switch to the CLI just to share a design doc, spec, RFC, or README with Claude.
• Common real-world inputs: PDFs (papers, specs), DOCX (requirements docs), and Markdown (READMEs, ADRs, design docs) are the most common document formats engineers want to hand to Claude Code.
• Discoverability: desktop users reasonably expect drag-and-drop to \"just work\" the way it does in the CLI and in the claude.ai web app.

Environment

• Claude Code desktop app (macOS)
• Compared against Claude Code CLI on the same machine

Additional notes

If there is a security / sandboxing reason certain file types are gated in the desktop app, it would help to either (a) surface a clear error message explaining this, or (b) expose a setting to allow-list additional file types.