Async file upload resets other controls

I have a set of regular controls (drop downs, text boxes, etc.) and an async file upload control, all inside an update panel.

When I select an item from drop down list and then upload a file, after file upload is complete the drop down gets reset to its original value; same thing with other controls.

In the onClientUploadComplete I have "__doPostBack("update_panel_ID"). If I remove this line, when file is being uploaded, I get "htmlfile: Unspecified error." followed by "Microsoft JScript runtime error: Permission denied"; it happens at this line "if(document.getElementById(this.mainDivId)==null".

Is there any way I can stop the other controls from getting reset? I know it is due to the __doPostBack(...) but if I remove it I get all these JS errors.

Thanks.