Hi again,
I just found out the solution and I think it's good to post it.
so basically the correct file to edit seems to be  share-documentlibrary-config.xml
I added the line:
 <create-content>
         <content id="plain-text" mimetype="text/plain" label="create-content.text" itemid="cm:content" icon="text"/>
         <content id="html" mimetype="text/html"  label="create-content.html" itemid="cm:content"/>
         <content id="xml" mimetype="text/xml" label="create-content.xml" itemid="cm:content"/>
        <!–CUSTOM–>
       <content id="xml" mimetype="text/xml" label="My Label" itemid="my:myobj"/>
 </create-content>