< Neo22s />

function checkExt(e) {//use in a form event or ina input
       value=e.value;
	if( !value.match(/\.(jpg)|(gif)|(png)|(bmp)|(pdf)$/) ){//here your extensions
		alert("wrong extension");	//actions like focus, not validate...
	}
	else {//right extension
		alert("nice!");	//actions
	}
}

<input id="Path" name="Path" type="file" />

function isPdf(e) {
if( e.value.match(/\.(pdf)$/) ){
	alert("pdf");	//action
      }
	else alert("no pdf");	//actions
}