File upload and convert

Hi Folks, 

I have a content type with TWO file fields, in which I will designate TWO PDFs. On upload completion I want to run something like: "exec(pdftk f1.pdf background f2.pdf output f1.pdf)". This will "brand" f1.pdf. 

I don't see any standard mechanism and so I'm not sure how to go about this. I have a general understanding, but that doesn't answer the question. I am sure I don't want to intercept all possible uploads; just the uploads from this specific content type. So, can I define a file upload hook which is scoped to this specific content type? I haven't seen anything that makes me believe I can. Can I create a custom file field to use any time I need to perform a conversion? Can my custom file field "extend" the existing file field, so I don't have to port improvements/bugfixes to my custom file field version? 

So many questions... (-: 

Thanks for the help, 

Chris.

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