#modal {
    padding: 10px;
    background: #eee;
}

<formid="frm"><!-- form can contain plenty of inputs --></form> 
<divid="modal"><h2>Modal</h2><divid="file-input-wrapper"><inputid="modal-file-input"type="file"name="file"/></div><br/><br/><inputid="submit-modal"type="submit"value="submit modal"/></div>

$('body').on('click', '#submit-modal', function(e) {
    e.preventDefault();
    var $form = $('#frm');
    var $fileInput = $('#modal-file-input');
    var $fakeInput = $fileInput.clone();
 
	$form.append($fileInput);
    $('#file-input-wrapper').append($fakeInput);
});