ASP.NET

<ajax:AsyncFileUpload ID="fuSlides" Width="400px" runat="server"
				OnClientUploadError="uploadError" 
				OnClientUploadStarted="StartUpload" 
				OnClientUploadComplete="UploadComplete" 
				CompleteBackColor="Lime"
				UploaderStyle="Modern" 
				ErrorBackColor="Red" 
				onuploadedcomplete="fuSlides_UploadedComplete" 
				UploadingBackColor="#66CCFF" />


 <script type="text/javascript" language="javascript">
	     function uploadError(sender, args) {
//	         alert("[" + lblStatus + "]"); return false;
	         document.getElementById('<%= lblStatus.ClientID %>').innerHTML = args.get_errorMessage();
	         alert(args.get_fileName() + ' could not be uploaded. ' + args.get_errorMessage());
		 }
	     function StartUpload(sender, args) {
//	         alert("[" + fuSlides + "]"); return false;
			 var filename = args.get_fileName();
			 var filext = filename.substring(filename.lastIndexOf(".") + 1);
			 if (filext == "doc" || filext == "docx" || filext == "pdf" || filext == "pages" || filext == "pptx" || filext == "ppt") {
				 return true;
			 }
			 else {
				 var err = new Error();
				 err.name = 'My API Input Error';
				 err.message = 'Only .ppt, .pptx, .doc, .docx, or .pdf  files';
				 throw (err);

				 return false;
			 }
		 }
	    function UploadComplete(sender, args) {
			var filename = args.get_fileName();
			var contentType = args.get_contentType();
			var text = "Size of " + filename + " is " + args.get_length() + " bytes";
			if (contentType.length > 0) {
			    text += " and content type is '" + contentType + "'.";
			    __doPostBack('UploadPostback', '');
			}
		}

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If you're sure about those two steps and still you get the same error, the simple workaround is to change the value of "ClientIDMode" to "AutoID".