mattermost: fix DM media upload for unprefixed user IDs (#29925)

Merged via squash.

Prepared head SHA: 5cffcb072c
Co-authored-by: teconomix <6959299+teconomix@users.noreply.github.com>
Co-authored-by: mukhtharcm <56378562+mukhtharcm@users.noreply.github.com>
Reviewed-by: @mukhtharcm

docs/automation/cron-jobs.md

@@ -262,6 +262,7 @@ If `delivery.channel` or `delivery.to` is omitted, cron can fall back to the mai
 Target format reminders:
 
 - Slack/Discord/Mattermost (plugin) targets should use explicit prefixes (e.g. `channel:<id>`, `user:<id>`) to avoid ambiguity.
+  Mattermost bare 26-char IDs are resolved **user-first** (DM if user exists, channel otherwise) — use `user:<id>` or `channel:<id>` for deterministic routing.
 - Telegram topics should use the `:topic:` form (see below).
 
 #### Telegram delivery targets (topics / forum threads)

docs/channels/mattermost.md

@@ -153,7 +153,14 @@ Use these target formats with `openclaw message send` or cron/webhooks:
 - `user:<id>` for a DM
 - `@username` for a DM (resolved via the Mattermost API)
 
-Bare IDs are treated as channels.
+Bare opaque IDs (like `64ifufp...`) are **ambiguous** in Mattermost (user ID vs channel ID).
+
+OpenClaw resolves them **user-first**:
+
+- If the ID exists as a user (`GET /api/v4/users/<id>` succeeds), OpenClaw sends a **DM** by resolving the direct channel via `/api/v4/channels/direct`.
+- Otherwise the ID is treated as a **channel ID**.
+
+If you need deterministic behavior, always use the explicit prefixes (`user:<id>` / `channel:<id>`).
 
 ## Reactions (message tool)