get thumbnail from myfiles

Using /2/files/get_thumbnail as you're doing here is the correct way to retrieve a thumbnail for a file from the Dropbox API. 

Per the documentation, you can specify a 'format' parameter to request either 'jpeg' or 'png' for the format of the thumbnail. You're not specifying that, so you'll get the default of 'jpeg'.

This is a "content-download" endpoint, so the HTTP response body itself will contain the requested data, in this case, the raw data for the thumbnail. In your code, that's available in the 'data' parameter given to the 'done' callback function. Exactly what you do with that data is up to you.

Also, note that we do offer and recommend using the official Dropbox API v2 JavaScript SDK instead. That has a corresponding filesGetThumbnail method that you can use instead of calling the endpoint directly.