Similar to Pinboards all files are visible to the specific conversation members. Anyone who's part of the conversation, can download the files and delete them if they should wish to do so.
Participants who have been there since the beginning of the conversation can see all the files in the conversation. People who join later will see the files when you've disclosed conversation history to them. If you do not disclose conversation history to new joiners then they will see only the files that were added after they joined the conversation.