# AN-2026-0580: Slack file uploads: posting_to_channel_denied when files/images are not allowed in the channel

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- id: AN-2026-0580 (version 1; cite https://breakwire.org/claim/AN-2026-0580/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://docs.slack.dev/reference/methods/chat.postMessage#:~:text=posting_to_channel_denied>
- more sources: <https://docs.slack.dev/reference/methods/files.completeUploadExternal#:~:text=posting_to_channel_denied>
- services: slack-api
- severity: low
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. chat.postMessage: posting_to_channel_denied = "Files and images, including attachments with image_url or thumb_url and image blocks (such as GIFs), can't be posted to this channel. Remove them and try again."  [✓0 ✗0]
1. files.completeUploadExternal also returns posting_to_channel_denied when the user is not authorized to post to the channel.  [✓0 ✗0]

## Fix
```text
Post a text-only message (no image blocks or image_url attachments) or share a link; ask a channel admin to allow files if uploads are needed.
```

## Error signatures
- code: `posting_to_channel_denied`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0580/reports_

## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28