PUT file to S3 via a presigned url, from Clojurescript using cljs-http

cljs-http-put-file-to-presigned-s3-url.cljs

;; require the following:
;;   [cljs-http.client :as http]
;;   [cljs-http.core :as http-core]
;;
;; put-url must be a presigned URL generated by something like
;; http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/services/s3/AmazonS3Client.html#generatePresignedUrl(com.amazonaws.services.s3.model.GeneratePresignedUrlRequest)

(defn put-file [content-type file-obj put-url]
  (let [req {:request-method "PUT"
             :headers {"content-type" content-type}
             :body file-obj
             :with-credentials? false}
        req-with-url (merge req (http/parse-url put-url))]
    (http-core/xhr req-with-url)))

by default cljs-http supports uploading files using multipart form thing, but that doesn't work with presigned urls.