`slack_file` image blocks appended with `chat.appendStream` render as missing images inline

Description

When an image block referencing a Slack-hosted file is appended to a streaming message using chat.appendStream, Slack initially renders a missing-image placeholder.

Clicking the missing image opens and displays the correct image, which indicates that:

• The file ID is valid.
• The upload completed successfully.
• Slack generated the image thumbnail.
• The original image remains accessible.

The same streamed message's DOM contains a valid Slack thumbnail URL, but Slack replaces the inline background image with an unable-to-load-error asset.

This may be a server-side file-sharing/association issue specific to blocks appended through the streaming APIs.

Reproducible in:

The Slack SDK version

3.43.0

Bot scopes:

• chat:write
• files:read
• files:write

Python runtime version

3.13.7

OS info

ProductName: macOS
ProductVersion: 26.6.2
BuildVersion: 25G83

Steps to reproduce:

(Share the commands to run, source code, and project settings (e.g., pyproject.toml))

1. Upload a PNG to Slack without posting a separate file message:

upload_response = await client.files_upload_v2(
    content=png_bytes,
    filename="sample.png",
)

file_id = upload_response["file"]["id"]

2. Wait until files.info reports that the image and its thumbnail are ready:

file_info = await client.files_info(file=file_id)
file = file_info["file"]

assert file["mimetype"] == "image/png"
assert file["filetype"] == "png"
assert file["thumb_360"]
assert file["thumb_360_w"]
assert file["thumb_360_h"]

3. Start a streaming message:

response = await client.chat_startStream(
    channel=channel_id,
    thread_ts=thread_ts,
)

stream_ts = response["ts"]

4. Append an image block using the Slack file ID:

await client.chat_appendStream(
    channel=channel_id,
    ts=stream_ts,
    chunks=[
        {
            "type": "blocks",
            "blocks": [
                {
                    "type": "image",
                    "slack_file": {
                        "id": file_id,
                    },
                    "alt_text": "Sample Image",
                }
            ],
        }
    ],
)

5. Stop the stream:

await client.chat_stopStream(
    channel=channel_id,
    ts=stream_ts,
)

Expected result:

The Slack-hosted image should render inline in the completed streaming message.

Actual result:

Slack renders a missing-image placeholder.
Clicking the placeholder opens and displays the correct image.
Inspecting the rendered HTML shows that Slack resolved the file to a generated thumbnail URL:

<a
  class="c-link c-message_attachment__image"
  href="https://files.slack.com/files-tmb/.../chart_480.png"
  style="
    background-image:
      url('https://a.slack-edge.com/.../unable-to-load-error-....png');
  "
>
  <span role="img" aria-label="missing image"></span>
</a>

The href points to the generated Slack thumbnail, while the inline background is replaced with Slack's unable-to-load-error image.

Requirements

For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

Start by reproducing the sequence with files_upload_v2, files_info, chat_startStream, chat_appendStream, and chat_stopStream using the versions listed. Compare the SDK request payload with Slack's streaming API behavior and determine whether the failure is in the SDK or server-side. Done means documenting the cause and adding a focused regression test if an SDK-side change is identified.