I changed the name of the account from the capitalized name, which was on disk, to lowercase, and that caused the problem. Reverting it back to capitalized fixed it.
("provider" ;; lower case!
(mu4e-sent-folder "/Provider/Sent Items")
It appears that mu4e uses its own file selection function, which is hard-coded. Which means you have to rewrite the functions in question after loading mu4e. The following code accomplishes this for adding attachments to outgoing files:
'(defun tws-attach-file (file &optional type description disposition)