download file API stopped working with 400 error

[Cross-linking for reference: https://stackoverflow.com/questions/42755495/dropbox-download-file-api-stopped-working-with-400-error ]

The error message is indicating that the access token isn't being supplied with the request properly. I can't offer much help with using the HTTP client in Angular, but here's a simplified version that works for me:

let headers = new Headers();
headers.append('Authorization', 'Bearer <ACCESS_TOKEN>');
headers.append('Dropbox-API-Arg','{"path": "/test.txt"}');
console.log(headers);
http.post('https://content.dropboxapi.com/2/files/download', null,
    new RequestOptions(
        {
            headers: headers,
            responseType: ResponseContentType.ArrayBuffer}
        )
    )
.subscribe((res) => {console.log(res)});

There were two main options:

1. You were passing the request options to the data parameter, instead of the config parameter of http.post.
2. You weren't passing in valid JSON as the 'Dropbox-API-Arg' value.