I am writing an asp.net web app which involves the use of the FileUpload control.
Right now, this particular FileUpload control only expects .z         
        
To select images in fileupload control..
hope it will help you
asp:RegularExpressionValidator ID="rexp" runat="server" ControlToValidate="fupProduct"
     ErrorMessage="Only .gif, .jpg, .png, .tiff and .jpeg"
     ValidationExpression="(.*\.([Gg][Ii][Ff])|.*\.([Jj][Pp][Gg])|.*\.([Bb][Mm][Pp])|.*\.([pP][nN][gG])|.*\.([tT][iI][iI][fF])$)">