Yammer web part not allowing file attachments

Using the Yammer Conversation web part, in a modern SharePoint page I cannot attach a file when I start a new discussion. The paper clip icon under the three dots/ellipsis is greyed out. 

 

I CAN attach a file when I post a reply to an existing discussion. 

 

According to the documentation , it should work in the web part.

 

• The new Yammer publisher functionality – When users create conversations, users have the following new features:

  • Create any type of Yammer post directly from SharePoint – questions, polls, praise, etc. 

  • Upload images and files into new conversations and replies directly from SharePoint

  • Use rich text in messages created from SharePoint.

What could be the problem?? I am a SharePoint admin, Yammer Admin, Global Admin, but regular SharePoint users and other Yammer group admins also can't attach files/images into a new discussion.