清空file域

<script language="javascript" type="text/javascript">
function allowExts(form){
	var fileObj = form.elements['weibo_image'];
	var file = fileObj.value;
	var fileName = file.substring(file.lastIndexOf("\\") + 1, file.length);
	var type = fileName.substring(fileName.lastIndexOf('.') + 1, fileName.length).toLowerCase();
	if ('|jpg|gif|png|jpeg|'.indexOf('|' + type + '|') == -1) {
		alert('上传图片格式不正确！');
		fileObj.outerHTML = fileObj.outerHTML;
		return;
	}
}
</script>