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

When developing web part from Microsoft.SharePoint.WebPartPages.WebPart class, we need to use dwp format, instead of .webpart format.

See: http://blog.sharepointdevelopment.nl/post/Unable-to-add-selected-Web-Part-Incompatible-Web-Part-markup-detected.aspx