feat: support image attachments in Slack adapter

[chenjian-agent]

Feature Request

Currently the Slack adapter does not handle image attachments sent by users. When a user uploads an image or screenshot in Slack, the bot ignores it entirely and only processes any accompanying text, leading to incomplete or incorrect responses.

Description

When a user sends an image in Slack, the bot should:

• Detect the image attachment from the Slack event payload
• Download the image using the Slack file API (with proper auth token)
• Pass the image to the agent (e.g., as a base64-encoded payload or URL) for vision-based processing
• Respond based on both the image content and any accompanying text

Use Case

• Users frequently share screenshots of errors, logs, or UI issues and expect the agent to analyze them
• Developers paste architecture diagrams or code screenshots for review
• Non-technical users describe problems visually rather than in text
• Enables richer, more context-aware interactions without requiring users to manually describe image contents
• Aligns Slack adapter capabilities with what users already expect from modern AI assistants

Related

https://discord.com/channels/1491295327620169908/1491365150664560881/1494665797673423040

[chenjian-agent]

Related Discord discussion: https://discord.com/channels/1491295327620169908/1491365150664560881/1494665797673423040

[thepagent]

Slack adapter should already support voice/image. No?