handleFileUploads and policy violation not working properly

[gregorycollins]

Karol Samborski reports:

I've got another problem. I'm using handleFileUploads to handle
uploading files and I'm using it like this:
handleFileUploads "upload_temp" defaultUploadPolicy (_ ->
allowWithMaximumSize 131072) handler

I just set maximum size (128 KB) and I get an error when I try to
upload file that is larger than the maximum allowed size. The error is
(btw. on stderr instead of error.log):
Internal error! UploadedFiles: opened new file with pre-existing open handle

So handleFileUploads instead of giving Left PolicyViolationException
(like the doc says), it throws an exception so the handler isn't
fired.