Stream gltf from HttpServlet to <web-viewer> element

I have a Tomcat running a JavaWebApplication within I want to use the <model-viewer> element (info here) to display a 3D model with 'changable' textures. Therefore I want to get the necessary gltf file from a Servlet, so I can take impact on it, rather than just accessing a file. I know, that both (the gltf structure as well as the model-viewer in my webapp) work, as it works flawlessly if i refer the src like this:

<model-viewer
    poster-color="black"
    poster="<%=
        "ressource?type=coloredmodelpic&key=" +
        model +
        "&ptype=" +
        type +
        "&color=" +
        color
        %>"
    camera-controls
    auto-rotate
    alt="<%= "A 3D model of " + model + "-" + type %>"
    src="data/RON-AA/RON-AA.gltf">

whereas obviously RON-AA.gltf holds the json structure. The call on the poster attribute also works flawlessly but when I want to use the same servlet (on which 'ressource' refers to) to stream the data it won't show the model.

<model-viewer
    poster-color="black"
    poster="<%=
        "ressource?type=coloredmodelpic&key=" +
        model +
        "&ptype=" +
        type +
        "&color=" +
        color
        %>"
    camera-controls
    auto-rotate
    alt="<%= "A 3D model of " + model + "-" + type %>"
    src="<%=
        "ressource?type=gltf&key=" +
        model +
        "&ptype=" +
        type +
        "&color=" +
        color
        %>"
>

I know the call itself is working, as when i directly call the url in any browser I do get the correct gltf to download. It also does provide the correct data as when I view the download in vim it seems to be alright. In the servlet I provide the data like (not exactly as it is spread upon a few methods, but overall like) this:

...
case "gltf":
    key = request.getParameter("key");
    ptype = request.getParameter("ptype");
    colorString = request.getParameter("color");
    response.setContentType("model/gltf+json");
    response.setHeader("Content-Type", "model/gltf+json");

    InputStream is;
    OutputStream os = response.getOutputStream();
    is = new FileInputStream(new File("...path to the same gltf... .gltf"));
    byte[] buffer = new byte[1024];
    int bytesRead;
    while ((bytesRead = is.read(buffer)) != -1) 
        os.write(buffer, 0, bytesRead);
        
    os.flush();
    os.close();
    is.close();

So I do set the correct(?) mimetype and do access the right 'template' to test the stream as the result in vim is the correct data. It even gets more confusing to me as when I try to stream the binary instead of the gltf for the same model-viewer element setup, it works. So the following is working just fine:

...
case "glb":
    key = request.getParameter("key");
    ptype = request.getParameter("ptype");
    colorString = request.getParameter("color");
    response.setContentType("model/gltf-binary");
    response.setHeader("Content-Type", "model/gltf-binary");

    InputStream is;
    OutputStream os = response.getOutputStream();
    is = new FileInputStream(new File("...path to the glb... .glb"));
    byte[] buffer = new byte[1024];
    int bytesRead;
    while ((bytesRead = is.read(buffer)) != -1) 
        os.write(buffer, 0, bytesRead);
        
    os.flush();
    os.close();
    is.close();[info here][1]

Could someone please give me a hint, what is going wrong as I am really out of ideas right now...