Hi Gonzalo,
The diagram of a process definition can be an image (jpeg, png, svg, gif) that must deployed along with the BPMN XML. By convention, it must have the same name as the BPMN XML but with a different suffix.
If you are interested in displaying a diagram in a web application, I suggest fetching the BPMN XML instead (via GET /process-definition/{id}/xml) and using bpmn.js to render it.
Cheers,
Thorben