Opportunistic networks are characterized by intermittent connectivity among mobile devices that occurs during their opportunistic contacts. With the increasing number of capable wireless devices and hence increasing potential formation of opportunistic networks, enabling applications over opportunistic networks has become critical. In this paper, we design and analyze a mobile file sharing system over opportunistic networks using Bluetooth technology. Our goal is to enable a mobile device to download desired files from available devices in its neighborhood and also allow its files to be disseminated to the network with the facilitation of opportunistic contacts. The key challenges we strive to address are system automation and file sharing efficiency. The mobile file sharing system is implemented on HP iPAQ 910 smart phones. Extensive experiments are conducted and the results indicate that our mobile file sharing system works efficiently and the data rate for file transferring can be up to 417 KB per second.