How use /get_thumbails response

It is indeed raw file data! However, in this case it works similarly to opening a JPEG image in notepad and copying it into another empty notepad file and renaming the file extension to JPEG - you may notice that the second file will not display an image and return an error! Depending on what media you are using to copy and paste this binary data, it may store it incorrectly then paste the corrupted data. 

I too had this same issue with many question marks showing up in the JFIF content-download returned from get_thumbnail API. I'm curious how it would work with DataURI but I was able to convert the returned JPEG/JFIF into an img tag by first converting it into a "blob" like below AND setting overrideMimeType to the "text/plain"

Below is a Javascript example using XMLHttpRequest that I used in my project:

const Http = new XMLHttpRequest();
const url = 'https://content.dropboxapi.com/2/files/get_thumbnail_v2';

//make sure that all text stays as plain text when read in...
Http.overrideMimeType('text/plain; charset=x-user-defined');

Http.open("POST", url);

Http.responseType = "blob";

Http.setRequestHeader('Authorization', 'Bearer ' + access_token_dropbox);

Http.setRequestHeader('Dropbox-API-Arg', JSON.stringify(data));

Http.send();

Http.onreadystatechange = (e) => {

if (Http.readyState === 4) {
let jpegData = Http.response;

var imageUrl = URL.createObjectURL(Http.response);

document.getElementById("firstDiv").innerHTML = `<img src="${imageUrl}" alt="jo mama" />`;
}}

I got this idea from another question posed here that had a similar issue and was resolved by changing xmlhttprequest.responseType to "blob"