Generate preview of attachments from the team folder

Currently, if I send an image from the team folder as an attachment to a team channel, it shows merely as a link. It would be nice if it showed a preview in the same way it would show as a regular attachment.

Start by tracing how attachments from a team folder are sent to a team channel and how regular image attachments are rendered. Compare the two paths and identify the client entry point responsible for the link-only behavior. Done means team-folder images render an inline preview like regular attachments.

Written by the indexing model from the issue text.