`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.

Comienza reproduciendo la secuencia con files_upload_v2, files_info, chat_startStream, chat_appendStream y chat_stopStream usando las versiones indicadas. Compara el payload de la solicitud del SDK con el comportamiento de la API de streaming de Slack y determina si el fallo está en el SDK o en el servidor. Se considera terminado cuando se documente la causa y se añada una prueba de regresión específica si se identifica un cambio del lado del SDK.