Efficient large file upload with Yesod

I want to implement large file upload with my Yesod application. Right now I have:

module Handler.File where

import Import

import System.Random
import System.FilePath
import Control.Monad
import qualified Data.ByteString.Lazy as LBS
import qualified Data.Text.Encoding

-- upload

uploadDirectory :: FilePath -- FIXME: make this configurable
uploadDirectory = "incoming"

randomFileName :: IO FilePath
randomFileName = do
  fname'base <- replicateM 20 (randomRIO ('a','z'))
  let fname = uploadDirectory </> fname'base <.> "bin"
  return fname

fileUploadForm :: Form (FileInfo, Textarea)
fileUploadForm = renderDivs $ (,)
    <$> fileAFormReq "Choose a file"
    <*> areq textareaField "What's on the file?" Nothing

getFileNewR :: Handler RepHtml
getFileNewR = do
  (formWidget, formEnctype) <- generateFormPost fileUploadForm
  defaultLayout $ do
       setTitle "Upload new file."
       $(widgetFile "file-new")

postFileNewR :: Handler RepHtml
postFileNewR = do
  user <- requireAuth
  ((result, formWidget), formEnctype) <- runFormPost fileUploadForm
  case result of
    FormSuccess (fi,info) -> do
                 fn <- liftIO randomFileName
                 liftIO (LBS.writeFile fn (fileContent fi))
                 let newFile = File (entityKey user) fn info (fileName fi) (fileContentType fi)
                 fid <- runDB $ insert newFile
                 redirect (FileViewR fid)
    _ -> return ()

  defaultLayout $ do
       setTitle "Upload new file."
       $(widgetFile "file-new")

It is mostly fine, except few issues:

Maximum size of a file is around 2 megabytes. I have a fix, but is it right way to do this? My fix is overriding default implementation of maximumContentLength method in Yesod instance for my app, like this:

maximumContentLength _ (Just (FileNewR _)) = 2 * 1024 * 1024 * 1024 -- 2 gigabytes maximumContentLength _ _ = 2 * 1024 * 1024 -- 2 megabytes
The amount of memory used is equal to the size of a file. This is really suboptimal. I would like to use tempFileBackEnd from http://hackage.haskell.org/packages/archive/wai-extra/1.2.0.4/doc/html/Network-Wai-Parse.html but I have no idea how to actually wire that into my request and make it work with forms logic (hidden _token field etc.).
The upload is 'single shot': any examples of how to make it work with Flash/Html5 based uploaders that show progress to user?

Your solution is correct. The purpose of the maximumContentLength setting is to allow you to override this value for specific routes that need larger uploads.
This is a shortcoming of the current setup of file handling in yesod-core. It's currently hard-coded to use in-memory file uploads. We've discussed on the mailing list in the past that this is suboptimal. I've just created a Github issue for this, and the fix will be included in Yesod 1.1 (no timetable on release though).
I don't have an example of this, sorry.