I am not familiar with restrictions on the encoding type when dynamically creating forms via the DOM, but here are some workarounds:
1. Dynamically create a form via HTML rather than the DOM ( var tempHTML = "<FORM method='post' enctype='adfjlsjf'>"; etc....) and write that to a dynamically created DIV.
2. Harcode a <FORM> tag in your page and try manipulating that via the DOM rather than creating a new form.
3. Use an AJAX submission