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