Using Regular Expressions to validate a filename in a FileUpload control

Here's a little code snippet I use to ensure that the file that has been uploaded is of type JPG, JPEG, PNG or GIF

// check anything has been uploaded
if (ThumbnailImageUpload.PostedFile.ContentLength > 0)
{
    // the regex for an image
    Regex imageFilenameRegex = new Regex(@"(.*?)\.(jpg|jpeg|png|gif)$");
 
    if (imageFilenameRegex.IsMatch(ThumbnailImageUpload.PostedFile.FileName, 
                            RegexOptions.IgnoreCase))
    {
        // we have a valid filename
        // .. do something ..
    }
}