Related: fdo#32553/fdo#31792 ensure correct mimetype

The flat xml filter also accepts .odt as an extension, so
looking up mimetype by extension could pick that up.

sw/source/ui/dbui/mmoutputpage.cxx

@@ -1045,8 +1045,17 @@ IMPL_LINK(SwMailMergeOutputPage, SendDocumentsHdl_Impl, PushButton*, pButton)
     String sExtension = lcl_GetExtensionForDocType(nDocType);
     switch( nDocType )
     {
-        case MM_DOCTYPE_OOO : break;
-        case MM_DOCTYPE_PDF : break;
+        case MM_DOCTYPE_OOO:
+        {
+            //Make sure we don't pick e.g. the flat xml filter
+            //for this format
+            pSfxFlt = SwIoSystem::GetFilterOfFormat(
+                String::CreateFromAscii( FILTER_XML ),
+                SwDocShell::Factory().GetFilterContainer() );
+        }
+        break;
+        case MM_DOCTYPE_PDF:
+        break;
         case MM_DOCTYPE_WORD:
         {
             //the method SwIOSystemGetFilterOfFormat( ) returns the template filter