This code system http://hl7.org/fhir/media-subtype defines the following codes:

CodeDisplayDefinition
diagramDiagramA diagram. Often used in diagnostic reports
faxFaxA digital record of a fax document
scanScanned DocumentA digital scan of a document. This is reserved for when there is not enough metadata to create a document reference
retinaRetina scanA retinal image used for identification purposes
fingerprintFingerprintA finger print scan used for identification purposes
irisIrisAn iris scan used for identification purposes
palmPalmA palm scan used for identification purposes
faceFaceA face scan used for identification purposes