Add custom property/attribute to new message

[GregAtFramework]

The Javascript docs for sending a message show that you can custom fields, e.g. anotherCustomField in the example.

How would you do this using the React Native UI components? Ideally I would be able to specify a function that takes the message, and allows me to transform it. (In my specific case I want to add a custom attribute that specifies whether the message has an attachment, in order to use this in my push notification template.)

(See also support request 21697.)