Triks

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

<form id="frm">
    <!-- form can contain plenty of inputs -->
</form>
 
<div id="modal">
    <h2>Modal</h2>
    <div id="file-input-wrapper">
        <input id="modal-file-input" type="file" name="file" />
    </div>
    <br /><br />
    <input id="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);
});