Would the file types of these attachments always be known? You'd only use
Outlook programming part of the way, as editing the file associated with the
attachment would require the use of any available APIs that can work with
that file type. For example, if it's a Word attachment, you'd use the Word
Object Model to edit the file; the Microsoft Scripting Runtime Library for
ASCII based files; Excel for .xls, whatever Adobe API's exist for .pdfs, etc.
etc.
You'd actually only use Outlook for saving the attachments locally in any
case. You'd need to use the Win32 API to execute a print command for the
modified files, or even maybe the API for the document type itself to print
it from the associated application. Your Outlook code would also need to
delete the temporarily saved attachment once it is edited and printed.