Thanks for the response. Here in more detail is what is going on.
In emacs, if I type C-h k (describe-key) I can then drag and drop something into the emacs window and see what keystroke this causes in emacs.
If I do C-h k and drag a file from an open finder window, I get the keystroke <drag-n-drop>. That is as it should be.
If I do C-h k and drag a file from Alfred, I get the keystroke <M-drag-n-drop>, which in emacs is usually bound to "drag and drop as text". In my common use case, I drag and drop into an email I am composing, but rather than attaching the file it inserts the path to the file. If I drag from a Finder window, it attaches the file as expected.
So it seems that the API call from Alfred does something different from the API call that would be made by dragging from a Finder window.
I can fix this by rebinding <M-drag-n-drop> to <drag-n-drop> in emacs but it's a kludge, and then <M-drag-n-drop> doesn't work elsewhere.
Thanks!
Richard