aContractID.AssignWithConversion(kTransformerContractIDPrefix); aContractID.Append(aMimeType);
aContractID.Assign(NS_LITERAL_CSTRING("@mozilla.org/document-transformer;1?type=")); aContractID.AppendWithConversion(aMimeType);
nsresult ConstructContractID(nsString& aContractID, const nsString& aMimeType){  aContractID.AssignWithConversion(kTransformerContractIDPrefix);  aContractID.Append(aMimeType);  return NS_OK;}