Form encoding is not set to MultiPart if the file fields are within a list (Snap/Heist)

[cimmanon]

I'm building a fairly simple form for managing images that contains 2 lists: uploaded images and existing images.

data Image = Image
	{ path :: FilePath
	, source :: Maybe Text
	} deriving (Eq)

imagesForm :: Monad m => Maybe [Image] -> Form Text m ([Image], [Image])
imagesForm xs = ( , )
	<$> "upload" .: listOf uploadForm Nothing
	<*> "update" .: listOf updateForm xs

uploadForm :: Monad m => Maybe Image -> Form Text m Image
uploadForm _ = validate hasFile $ ( , )
	<$> "path" .: file
	<*> "source" .: optionalText Nothing
	where
		hasFile (Just p, s) = Success $ Image p s
		hasFile (_, s) = Error $ "No file provided: " <> fromMaybe "no source provided" s

updateForm :: Monad m => Maybe Image -> Form Text m Image
updateForm x = Image
	<$> "path" .: string (path <$> x)
	<*> "source" .: optionalText (source =<< x)

The rendered form has enctype="application/x-www-form-urlencoded", so no files get uploaded. If I manually set the enctype on the form to be multipart/form-data, all of the POST data is ignored and the submitted form always fails, with failure state containing the form's defaults. Checking the contents of POST from Snap gives me an empty list.

If I add a dummy file field to my form, one that's not within a list, the form is correctly set to multipart and the form works as expected.

imagesForm :: Monad m => Maybe [Image] -> Form Text m ([Image], [Image])
imagesForm xs = (\a b c -> (a, b))
	<$> "upload" .: listOf uploadForm Nothing
	<*> "update" .: listOf updateForm xs
	<*> "foo" .: file