A multipart storage engine for storing uploads in temporary files.
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