Media Items are gained by hacking portals, they can be an Ingress report, anomaly status report, or a story line video usually with clues hidden within that you can decode to get an Ingress passcode. Media items are usually text, image, video or audio, top secret Niantic documents that have somehow been leaked. Every week an Ingress Report is also leaked to through Media items.. Most of the media are already solved and the passcodes are redeemed, so Google the name or URL of the item before you try to decode it, there is a high chance that it is already done. Lately Ingress Report codes have not had a redemption limit imposed on them so the codes stay alive forever.
All media have a level from 1 to 8, and there is a higher chance for portal to drop media that have same or lower level than portal.