When I try to forward any video clips that I downloaded to my phone from any other contact it sends immediately (ie without any uploading) . Also attaching some mp3 files are also sent instantly without showing any delay in uploading? How is this possible? I think they are storing all media files on their server.
I guess that's because whatsapp stores attachments on their database with its own checksum (every file can be represented by an alphanumeric sequence of characters called like this).