$ _FILES数组有时是空的

i have an issue with sending files form in magento CE 1.9.2.1. When i trying to attach files in php $_FILES array is empty but sometimes not. Please help me to find solution. Here is html form:

<form action="<?php echo $this->getUrl('quote/index/save', array('_secure'=>true)); ?>" id="get_a_quote" method="post" name="get_a_quote" enctype="multipart/form-data">
    <div id="wizard">
        <label for="attachment">Attachment</label>
        <input type="file" name="attachment[]" multiple="multiple" />
        <input id="submit" type="submit" name="save" value="Get A Quote" />
    </div>
</form>

in IndexController my save action code is:

print_r($_FILES); die;// result Array() but sometimes Array(bla bla bla)

My server info is:

post_max_size = 8M
upload_max_filesize = 16М