Clojure: map as function parameter -

i'm trying running codes book "web development clojure". there function can not understand:

(defn handle-upload [{:keys [filename] :as file}]   (upload-page      (if (empty? filename)       "please select file upload"       (try          (upload-file (gallery-path) file)         (save-thumbnail file)         (db/add-image (session/get :user) filename)         (image {:height "150px"}           (str "/img/"                (session/get :user)                "/"                thumb-prefix                (url-encode filename)))         (catch exception ex            (str "error uploading file " (.getmessage ex))))))) 

where

(defn upload-page [info]     (layout/common       [:h2 "upload image"]       [:p info]       (form-to {:enctype "multipart/form-data"}                [:post "/upload"]                (file-upload :file)                (submit-button "upload")))) 

what meaning of parameter of function handle-upload?

and after changing from

(defn handle-upload [{:keys [filename] :as file}] ...

to

(defn handle-upload [{:keys filename :as file}] ...

i got error message:

java.lang.illegalargumentexception: don't know how create iseq from: clojure.lang.symbol, compiling:(picture_gallery/routes/upload.clj:32:1)

why?

{:keys [filename] :as file} means:

1. take :filename key passed argument , bind value filename
2. leave whole argument available file

so if pass:

{:filename "foo"  :somethingelse "bar"} 

as argument, filename in function scope equal foo , file equal whole hash map.

references:

• special forms