expression evaluation error at function fn!content_appian_internal Null argument are invalid

Hi [mention:126a676c85024855948336691b0a7b92:e9ed411860ed4f2ba0265705b8793d05] ,

Thanks for redirecting me to this link very helpful. But here is another situation.

Let me explain my scenario,

I have used file upload in interface  and currently saving document into the cdt fields named brokerConfirm & otherDocumentation & both these fields are of type text..

what i am thinking now that i have to create another rule input and its type will be document
and then i can use this rule input in order to save the dcoument first before saving it to the actual CDT field. 

Please let me know is it right approch ??