Performs a lookup of registered #GMimeObject subclasses, registered
using gmime.object.ObjectWrap.registerType, to find an appropriate class
capable of handling MIME parts of type type/subtype. If no class
has been registered to handle that type, it looks for a registered
class that can handle type. If that also fails, then it will use
the generic part class, #GMimePart.
Performs a lookup of registered #GMimeObject subclasses, registered using gmime.object.ObjectWrap.registerType, to find an appropriate class capable of handling MIME parts of type type/subtype. If no class has been registered to handle that type, it looks for a registered class that can handle type. If that also fails, then it will use the generic part class, #GMimePart.