block/type/image should allow image_url to reference an uploaded file to Slack without having to make it public

(Describe your issue and goal here)

This should work:

1. upload an image with client.files_upload_v2(), do not want it to be visible in a channel
2. chat_postMessage(blocks=type: image, image_url: file.private_url ...)

This currently doesn't work unless the file/image is made public via client.files_sharedPublicURL(), but enabling public files is not an option in many organisations. Additionally one shouldn't have to construct urls by extracting pub_secret from file/permalink_public.

Given that slack client can show uploaded images (even when they are not public), it should also work with blocks/type/image.

Example discussion/solution (with making the file public) - https://stackoverflow.com/questions/58186399/how-to-create-a-slack-message-containing-an-uploaded-image

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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 en slack_sdk.models, centrándote en el constructor del bloque de imagen y en cómo se representa su valor image_url. Verifica el comportamiento con un archivo de Slack subido de forma privada y considera completado el trabajo cuando el bloque pueda hacer referencia a ese archivo sin hacerlo público ni extraer manualmente una URL pública.