datatransfer: automatically accepts images instead of awaiting

+ When someone send an images with a size < 20Mb, accept it
+ Note: add some unit tests for data transfer
+ clients must define downloadDirectory to activate this function

Change-Id: I27557d6c0fe5c218de875451376fde768b9ff972
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>