Filter files in fileupload control open dialog by file extensions using javascript?

Note: As like windows .Net its not possible to show only specific file extensions,the only way is that we can validate it by manually as shown below.

function checkFileExtension(elem)
{
var filePath = elem.value;

if(filePath.indexOf('.') == -1)
return false;

var validExtensions = new Array();
var ext = filePath.substring(filePath.lastIndexOf('.') + 1).toLowerCase();

validExtensions[0] = 'pdf';
//validExtensions[1] = 'jpg';
//validExtensions[2] = 'txt';

for(var i = 0; i < validExtensions.length; i++)
{
if(ext == validExtensions[i])
{

return true;
}
}

alert('The file extension ' + ext.toUpperCase() + ' is not allowed,select PDF.');

return false;
}


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FileUpload1.Attributes.Add("onchange", "return checkFileExtension(this);");