Override GPX mime type

app/src/main/java/eu/faircode/email/EntityAttachment.java

@@ -188,6 +188,9 @@ public class EntityAttachment {
                 type = gtype;
         }
 
+        if ("gpx".equals(extension))
+            return "application/gpx+xml";
+
         if ("eml".equals(extension))
             return "message/rfc822";