registering new MIME types for an application

I'm not sure if this is the correct list for this (there are tons!),  
but it was suggested that I try this list. Here's my question:

We have recently been asked by one of our customers if we have  
registered MIME types for our application's documents. They said they  
needed MIME types because part of their process is web-based, and  
suggested we use the following assignments:

application/vnd.multiad.creator
image/vnd.multiad.creator

MultiAd is our company name. Creator is the name of our software  
application which creates and edits documents. The Creator file  
format is proprietary. While Creator documents are of a graphical  
nature, I don't feel they are "image" documents in the same vein that  
the current

I've read through RFC4288, but am still unsure if we should register  
only the application/vnd.multiad.creator or both the appliation and  
image types. I'm very new to this whole MIME type process (I'm a  
developer, not an IT type person). Would you be able to suggest what  
would be best, or explain why both might be required?

_________________________________________________________
Steve Mills                              Me: 952-401-6255
Senior Software Architect                         MultiAd
[email protected]                       www.multi-ad.com