Naven
Glossary
File sharing
Sending a document, image or video inside a conversation.
The hard parts are size limits, storage cost and safety: an attachment is the most common delivery route for malware, and an image is the most common route for content that should not be there.
Scanning attachments requires the server to read them, which is the same trade encryption forces everywhere else.
Where Naven stands
Naven screens uploaded images before they are delivered, by two independent checks that both have to pass.