Example for figuring out how file upload works.

handler.clj

(ns file-upload.handler
  (:require [compojure.core :refer :all]
            [compojure.route :as route]
            [ring.middleware.defaults :refer [wrap-defaults site-defaults]]
            [ring.middleware.params :refer [wrap-params]]
            [ring.middleware.multipart-params :refer [wrap-multipart-params]]
            [ring.middleware.reload :refer [wrap-reload]]
            [ring.adapter.jetty :refer [run-jetty]]
            [hiccup.core :refer [html]]))

(defn home-page []
  (html [:form {:action "/file" :method "post" :enctype "multipart/form-data"}
         [:input {:name "file" :type "file" :size "20"}]
         [:input {:type "submit" :name "submit" :value "submit"}]]))

(defn upload-file [file]
  (let [file-name (file :filename)
        size (file :size)]
    (do
      (prn file)
      {:status  200
       :headers {"Content-Type" "text/html"}
       :body    (html [:h1 (str file)]
                      [:h1 size])})))
(defroutes
  app-routes
  (GET "/" [] (home-page))
  (POST "/file" {params :params}
    (let [file (get params "file")]
      (upload-file file)))
(route/not-found "Not Found"))

(def app
  (-> app-routes
      wrap-params
      wrap-multipart-params
      wrap-reload))

(defonce server (run-jetty #'app {:port 8080 :join? false}))