In one of my projects, I had an requirement where when a user browse a file via "FileUpload" control and hit the "Submit" button then button should be disabled. Moreover, I thought besides disabling the button I could display an image as well so that the user interface is more interactive. I have developed a custom user control that is hosted in an application page.
<asp:button id="btnOne" tabIndex="0" Runat="server" Text="Submit" onclick="btnOne_Click" OnClientClick="disableBtn(this.id, 'Submitting...')" UseSubmitBehavior="false" />
This code works perfectly on Firefox but image loading part did not work.
There is a very useful tip from 4guysfromrolla where page "Freeze" on the form submission.