foo.el

;; http://www.emacswiki.org/cgi-bin/wiki/http-post-simple.el

(http-post-simple-multipart "http://www.tumblr.com/api/write" 
                            (list (cons 'caption "foo")
                                  (cons 'type "photo")
                                  (cons 'email "something") 
                                  (cons 'password "something")
                                  (cons 'group "testingtumble.tumblr.com"))
                            (list "data" "avatar.png" "image/png" (file-data "/home/federico/avatar.png")))

(defun file-data (filename)
  (with-temp-buffer
    (insert-file-contents-literally filename)
    (buffer-substring-no-properties (point-min)
                                    (point-max))))

;; Error
Debugger entered--Lisp error: (wrong-type-argument listp "data")
  cdr("data")
  (setq --cl-rest-- (cdr --cl-rest--))
  (prog1 --cl-rest-- (setq --cl-rest-- (cdr --cl-rest--)))
  (car (prog1 --cl-rest-- (setq --cl-rest-- ...)))
  (pop --cl-rest--)
  (if (= (length --cl-rest--) 4) (pop --cl-rest--) (signal (quote wrong-number-of-arguments) (list nil ...)))
  (let* ((--cl-rest-- file) (fieldname ...) (filename ...) (mime-type ...) (data ...)) (http-post-bound-field (format "Content-Disposition: form-data; name=%S; filename=%S" fieldname filename) (format "Content-type: %s" ...) "" data))
  (progn (let* (... ... ... ... ...) (http-post-bound-field ... ... "" data)))
  (destructuring-bind (fieldname filename mime-type data) file (http-post-bound-field (format "Content-Disposition: form-data; name=%S; filename=%S" fieldname filename) (format "Content-type: %s" ...) "" data))
  (lambda (file) (destructuring-bind (fieldname filename mime-type data) file (http-post-bound-field ... ... "" data)))("data")...