Fixed out by one error

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@21982 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/unix/mimetype.cpp

@@ -1821,7 +1821,7 @@ int wxMimeTypesManagerImpl::AddToMimeData(const wxString& strType,
 
         // change nIndex so we can use it below to add the extensions
         m_aExtensions.Add(wxEmptyString);
-        nIndex = m_aExtensions.size();
+        nIndex = m_aExtensions.size() - 1;
 
         m_aDescriptions.Add(strDesc);
     }