allow multiple types in `add_file()` or add documentation about type manipulation

[mashehu]

According to the "Main Workflow Diagram" section in https://about.workflowhub.eu/Workflow-RO-Crate/ the workflow diagram should have types ["File", "ImageObject"], however using add_file() automatically assigns only type File.

Is there a way how I can add this additional type either during add_file() or afterwards?

[simleo]

diagram = crate.add_file("diagram.png", properties={"@type": ["File", "ImageObject"]})