For a web app, I wanted to run a quick check via javascript to make sure the file being uploaded was in the allowed file types (obviously this was hard-checked server side also), just to save the user some time and not need to process the whole form because of a simple filename mistake. Basic working example here.

<script type="text/javascript">

    function checkExtension()
    {
   
        // for mac/linux, else assume windows
        if (navigator.appVersion.indexOf('Mac') != -1 || navigator.appVersion.indexOf('Linux') != -1)
            var fileSplit = '/';
        else
            var fileSplit = '\\';
   
        var fileTypes     = new Array('.gif', '.jpg'); // valid filetypes
        var fileName      = document.getElementById('myInput').value; // current value
        var extension     = fileName.substr(fileName.lastIndexOf('.'), fileName.length);
        var valid = 0;
       
        for(var i in fileTypes)
        {
       
            if(fileTypes[i] == extension)
            {
           
                valid = 1;
                break;
               
            }
       
        }
       
        if(valid == 1)
            alert("Valid filetype, huzzah!");
        else
            alert("Do not collect 200 dollars!");
       
    }
   
</script>

<input type="file" id="myInput" />
<input type="button" value="Check File" onclick="checkExtension();" />