Allow users to upload files and attach to specific nodes Here's what I'd like to do:

I have nodes A, B, C, and D

I want users to be able to upload files using a file field on node D, but then for those files to actually show on nodes A, B, or C, depending on what the user specifies during the upload process. So node D is just acting as an upload form of sorts -- it wouldn't show any of the uploaded files.

Is this possible to do with existing modules?
In a word, No. 

At least not the way you are asking the question...

You need to be thinking about node types as well as just A, B, C &amp; D.

I would look into the views module (tutorial here).

This would mean you could upload your files to nodes of a particular type, and then create several views that display different files uploaded based on your custom parameters.

It's a bit hard to be any more specific based on the broadness of your question.