There’s a file upload endpoint that you’ll want to upload your file to. Good discussion of how to interact with it at the end of this thread:
I’m not able to tell from your example code where exactly your file is going, if anywhere.
Aside: You don’t need to include everything in the PUT request, and you probably specifically do NOT want to include the mobiledoc (phased out). You should be able to include the id or slug, updated_at, and the feature_image only.