Fix missing file attachment in Conpherence

Summary:
When you drag and drop a file in a Conpherence chat,
the file was only visible by the author.

Now the file is also attached to that chat, making it visible.

This is a follow-up from:

https://we.phorge.it/D25705?id=2178

Refs T15106

Test Plan:

1. open a Conpherence chat or create a new one
2. drag and drop file in it and send the message
3. verify file is attached to the chat

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: avivey, Cigaryno, Matthew, valerio.bozzolan, tobiaswiese

Maniphest Tasks: T15106

Differential Revision: https://we.phorge.it/D25709