[Minor] Add Windows Metafile MIME types (RFC 7903)

conf/maps.d/mime_types.inc

 image/vnd.valve.source.texture 0
 image/vnd.xiff 0
 image/vnd.zbrush.pcx 0
+image/wmf 0
+image/x-wmf 0
 message/CPIM 0
 message/delivery-status 0
 message/disposition-notification 0

lualib/lua_mime_types.lua

     {"wm", "video/x-ms-wm"},
     {"wma", "audio/x-ms-wma"},
     {"wmd", "application/x-ms-wmd"},
-    {"wmf", "application/x-msmetafile"},
+    {"wmf", {"application/x-msmetafile", "image/wmf", "image/x-wmf"}},
     {"wml", "text/vnd.wap.wml"},
     {"wmlc", "application/vnd.wap.wmlc"},
     {"wmls", "text/vnd.wap.wmlscript"},
     ["video/x-msvideo"] = "avi",
 }
 
-return exports
+return exports