Bug Description

Hermes cannot send file attachments to Mattermost. When the agent tries to attach a file to a Mattermost message — an image it generated, a document a tool produced, a screenshot, anything — the text part of the message goes through but the file is silently dropped. The bot returns a warning saying Mattermost is not in the supported list, even though the Mattermost adapter clearly has working upload code.

Steps to Reproduce

1. Configure the Mattermost gateway with a working bot token and a channel/DM the bot can post in.
2. Ask Hermes to send you a file in Mattermost — for example: "generate a small PNG and send it to me in Mattermost", or "send me the file /tmp/foo.pdf in Mattermost".
3. Hermes invokes send_message with a MEDIA:<path> tag in the message body.
4. The text arrives in Mattermost; the file does not.

Expected Behavior

The file should arrive in Mattermost as a native attachment on the bot's message, the same way it does on Telegram, Discord, Matrix, Signal, Feishu, etc.

Actual Behavior

The message arrives in Mattermost with text only — no attachment. The send_message tool result includes the warning: MEDIA attachments were omitted for mattermost; native send_message media delivery is currently only supported for telegram, discord, matrix, weixin, signal, yuanbao and feishu
If the agent tries to send a media-only message (no text), the tool returns an error and posts nothing: send_message MEDIA delivery is currently only supported for telegram, discord, matrix, weixin, signal, yuanbao and feishu; target mattermost had only media attachments
So from a user's perspective, the bot simply cannot share files on Mattermost.

Affected Component

Gateway (Telegram/Discord/Slack/WhatsApp)

Messaging Platform (if gateway-related)

No response

Debug Report

N/A — reproducible from any `send_message` call to Mattermost containing a `MEDIA:` tag.

Operating System

Debian 13

Python Version

3.13.5

Hermes Version

v0.13.0

Additional Logs / Traceback (optional)

Root Cause Analysis (optional)

No response

Proposed Fix (optional)

No response

Are you willing to submit a PR for this?

• I'd like to fix this myself and submit a PR