Returns a function that stores multipart file parameters as temporary files.
Accepts the following options:

:expires-in - delete temporary files older than this many seconds
              (defaults to 3600 - 1 hour)

The multipart parameters will be stored as maps with the following keys:

:filename     - the name of the uploaded file
:content-type - the content type of the upload file
:tempfile     - a File object that points to the temporary file containing
                the uploaded data
:size         - the size in bytes of the uploaded data