Teams channel file attachments fixed

sudie-codes

·Apr 12, 2026·#64645fix(msteams): channel file attachments broken by overly-broad HTML fallback (#58617, #51749)

Teams channel file attachments were silently failing — uploads reached the bot but delivered nothing, while false Graph API calls generated misleading error logs for every @mention message.

When users attached files to Teams channel messages, the bot saw the text but never received the file content. The system reported valid permissions and successful fetches, yet returned empty every time — a silent content loss masked by empty error handlers.

The root cause was a detection flaw: the code treated every text/html attachment as a file candidate, triggering the Graph API fallback for @mention cards and other HTML chrome that aren't actual files. When the fallback fired, it called ${messageUrl}/attachments — a Graph API sub-resource that doesn't exist for channel messages, returning 404s that were swallowed by empty catch blocks.

File attachments in Teams channels now work correctly. The fallback only triggers when real <attachment id> tags are present, and attachments are sourced from the correct location in the Graph response. Failures surface as logged warnings instead of disappearing silently.

View Original GitHub Description

Summary

Channel file attachments (users uploading files into a Teams channel conversation) were silently failing because the Graph media fallback was triggering on any HTML attachment — including @mention cards — and calling a nonexistent Graph sub-resource.

Root cause

hasHtmlAttachment in inbound-media.ts treated all text/html attachments as file candidates, including mention cards
Graph fallback path called \${messageUrl}/attachments — a sub-resource that does not exist in the Graph API
Errors were swallowed by empty catch {} blocks, masking the real failure

Fix

Gate fallback on presence of <attachment id="..."> tags in the HTML content
Source attachments from the chatMessage.attachments array returned by the main message fetch (correct Graph path)
Replace empty catch {} with proper error logging

Fixes #58617 Fixes #51749

Test plan

@mention cards no longer trigger fallback
Real file attachments route through the correct Graph path
Errors are now logged instead of swallowed
Manual: upload a file to a Teams channel with the bot

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