More flexibility in dino file transfers

Resource-wise, messenger applications tend to be on the lightweight side of the spectrum. This drastically changes when file transfers are added to the equation. File transfers can introduce arbitrary more resource-usage, both on network and data storage aspects. To alleviate this issue, stateless file sharing empowers the user to make informed decisions on which files to load. Deliverables • Unified handling of http and jingle (peer-to-peer) file transfers • Enable sending metadata alongside files • Thumbnail previews for images