<html>
<head>
<script type="text/javascript" language="javascript">
function valid(ele)
{
var file_name= ele.file.value;
var file_array = file_name.split(".");
var file_array1 = file_array[1].toLowerCase();
if (file_array1 == 'gif' || file_array1 == 'jpg' || file_array1 == 'jpeg',file_array1=='gif')
{
return true;
}
else {
alert("Please Upload Valid Image File");
ele.file.focus();
return false;
}
}
</script>
</head>
<body>
<form name="form1" action="upload_file.php" method="post" enctype="multipart/form-data" onsubmit="return valid(this);">
<label for="file">Filename:</label>
<input type="file" name="file" id="file" />
<br />
<input type="submit" name="submit" value="Submit" />
</form>
</body>
</html>
翻译自: https://bytes.com/topic/javascript/insights/904261-how-get-file-extension-using-javascript