You cannot couldn't. FormData
is was unfortunately not enumerable.
However, as you say only one of your forms does contain a file input. Then it should be possible to use serializeArray
on the other and append
to the data manually:
var formData = new FormData(document.forms['form-ship']); // with the file input
var poData = jQuery(document.forms['po-form']).serializeArray();
for (var i=0; i<poData.length; i++)
formData.append(poData[i].name, poData[i].value);