How WhatsApp send audio files or video clips instantly? 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).
When you try to upload a file whose checksum is already on their database, they just use the uploaded copy of the file. 
This leads to several advantages. Indeed, preventing double uploads:


Avoids server overloads
Makes you save mobile data
Make whatsapp save space on their database