Summary

The Discord adapter does not handle message attachments. DiscordGatewayMessage has no attachment fields, DiscordNetGatewayClient.OnMessageReceivedAsync ignores message.Attachments, and all inbound messages arrive with hasMedia=False. Users sending images or files in Discord get no indication their attachments were received.

Current State

• DiscordGatewayMessage (in DiscordTransportContracts.cs) has no attachment/media fields
• DiscordNetGatewayClient.OnMessageReceivedAsync extracts only message.Content (text)
• DiscordSessionBindingActor always sends hasMedia=False to the session pipeline
• ChannelAttachments config exists in audience profiles but has no effect since attachments are never extracted

Expected Behavior

Match Slack's attachment ingress pipeline:

• Extract message.Attachments from Discord.Net's SocketUserMessage
• Validate against ChannelAttachments.AllowedCategories and MaxFileBytes for the session's audience
• Download attachments to the session media directory
• Include attachment metadata in ChannelInput so the LLM receives them
• Support images, PDFs, documents, and other file types based on audience config

Reference Implementation

The Slack adapter has a complete attachment pipeline to mirror:

• SlackThreadBindingActor — attachment ingress handling, download, and ChannelInput assembly
• SlackFileFlowIntegrationTests.cs — comprehensive test coverage for file ingress scenarios
• ChannelAttachmentPolicy — shared validation logic for attachment categories and size limits

Test Coverage Required

Following the Slack test patterns in SlackFileFlowIntegrationTests.cs:

• Image attachment accepted for allowed audience
• Attachment rejected when category not in AllowedCategories
• Attachment rejected when exceeding MaxFileBytes
• Multiple attachments in a single message
• Attachment with text content (mixed message)
• Attachment-only message (no text)
• Attachment download failure degrades gracefully

Discovered During

Live Discord integration testing — sent an image in a public channel thread, session logged hasMedia=False and bot had no awareness of the attachment.