To restrict File upload control to accept only specified types to be uploaded using Java Script

Here is the code to restrict our file upload control to accept only specified file types to be uploaded,using Javascript.

In this example i am allowing only image files of type ".jps,.jpeg,.png,.gif"

JavaScript Code:
File Upload Control:

<input type="file" id="uploadImg" name="upload" onchange="checkfile(this);" />

function checkfile(sender)
    {
        var validExts = new Array(".jpg", ".JPG", ".jpeg", ".JPEG", ".gif", ".png", '.PNG');
        var fileExt = sender.value;
        fileExt = fileExt.substring(fileExt.lastIndexOf('.'));

        //For Indexof() -IE < 9
        if (!Array.prototype.indexOf)
        {
            Array.prototype.indexOf = function (elt /*, from*/)
            {
                var len = this.length >>> 0;
                var from = Number(arguments[1]) || 0;
                from = (from < 0) ? Math.ceil(from) : Math.floor(from);
                if (from < 0)
                    from += len;
                for (; from < len; from++)
                {
                    if (from in this && this[from] === elt)
                        return from;
                }
                return -1;
            };
        }
        if (validExts.indexOf(fileExt) < 0)
        {
            alert("Invalid file selected, valid files are of " +
               validExts.toString() + " types.");
            sender.value = '';
            return false;
        }
        else
        {
            return true;
        }
    }