At the moment the Attach a file option in the chat composer menu does not function while the browser window is sized at a desktop-appropriate width.
The expectation is that this would bring up the native file chooser and allow you to pick a file to attach to your chat message.
If you reduce the width of your browser window enough to change the UI to the typical “mobile” experience, the behavior functions normally.
The issue is limited to desktop Safari, there are no errors in the browser console, and dragging and dropping a file works fine.