Should the Save/Cancel buttons determine whether changes made to the file attachments are saved or discarded? Or would users expect that these changes are automatically and independently saved, since they entail file operations?
It actually depends on how you will display these changes.
Apply/Cancel behaviour should differentiate applied changes from changes which aren't yet saved thus helping user to understand the workflow (modify then apply/save, unsaved changes will be lost, etc).
Or, otherwise, you may apply all the changes immediately and provide ability to undo operations (no save/cancel button; gmail uses this approach).
So, in a case you're going with "Apply/Cancel" approach you should think about how to show your users that some changes aren't yet applied (activating Apply/Save buttons and marking file attachments as "new" or "unsaved", etc should be enough).
I recommend you to join details and file attachments at the single screen like it's done, for example, at gmail, so your users will be aware of all the changes (including file attachments) from the first sight: