If you're using a server control's OnClientClick event to call SlickUpload_Submit(), you need to append ";return false;" to the end of the event so the page doesn't post back immediately. ASP.NET automatically uses a submit button type or adds code to post back the page to the end of the click handler, so returning false will keep that from happening. It would look like:

OnClientClick="SlickUpload_Submit();return false;"

Or, you can just use a regular input like:

<input type="button" value="Submit" onclick="SlickUpload_Submit()" />