mime: handle non-mime data, just pass it through

OpenKeychain/src/main/res/values/strings.xml

@@ -1363,6 +1363,7 @@
     <string name="msg_data_mime_length">""</string>
     <string name="msg_data_mime">""</string>
     <string name="msg_data_mime_ok">""</string>
+    <string name="msg_data_mime_none">""</string>
     <string name="msg_data_mime_part">""</string>
     <string name="msg_data_mime_type">""</string>
     <string name="msg_data_ok">""</string>