Incompatible Web Part markup detected. Use *.dwp web part XML instead of *.webpart web part xml.

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This error can show up when you try to add a custom web part to a page. It happens when you have built a web part which uses a .webpart file, but where your web part class inherits from Microsoft.SharePoint.WebPartPages.WebPart. In order to use a .webpart file, your web part must inherit from System.Web.UI.WebControls.WebParts.WebPart.

Retrieved from "http://sharepointkb.org/Incompatible_Web_Part_markup_detected._Use_%2A.dwp_web_part_XML_instead_of_%2A.webpart_web_part_xml."