File attachments arrive with empty content in Desktop chat (Windows) — both drag-and-drop and file picker affected

What's Wrong?

Files attached to a regular chat in Claude Desktop on Windows arrive with zero content on the model side. The attachment chip appears normally in the UI (filename shown), but the extracted document content delivered to the model is completely empty.

Confirmed by two users independently on two Windows machines, with both attachment methods: drag-and-drop and the + / paperclip file picker.

Not file-type specific. Reproduced with:

• a 1.3 MB XHTML file (EUR-Lex document; valid, opens fine locally)
• a 36-byte plain test.txt containing a single line of text

Not model specific: reproduced with two different Claude models in the same conversation (Claude Fable 5, then switched mid-conversation to Claude Fable 5.1).

Still reproducible after updating Claude Desktop from 1.40609.1 to 1.44121.1 on 2026-09-02.

Counter-check: the Filesystem MCP server reads the exact same files from disk without any problem, so the files are intact. The defect is in the client's upload / content-extraction path.

What Should Happen?

The content of an attached file should be readable by the model, exactly as it is when the same file is read via the Filesystem MCP.

Steps to Reproduce

1. Claude Desktop on Windows, regular chat (not Cowork, not Code).
2. Create test.txt containing one line of text.
3. Attach it via drag-and-drop or the + button and send the message.
4. Ask the model what the file contains.
5. Model receives the attachment with 0 characters of content.

Reproduced 6 times in a single conversation across two file types, both attachment methods, two models, and before/after a client update.

Error Messages/Logs

None. The failure is silent — no error in the UI, the attachment simply has no content.

Environment

• OS: Windows 11 Pro 25H2 (Build 26200.8457)
• Claude Desktop version: 1.40609.1 and 1.44121.1 (reproduced on both; updated on 2026-09-02)
• Claude model: Claude Fable 5 and Claude Fable 5.1
• Regression? Unknown — we primarily use Claude Code, so we cannot say when this started.

Related issues

• #59311 — drag-and-drop files missing from Cowork uploads folder (Windows)
• #77946 — paste-converted attachments arrive empty (macOS)