Electron ES6 module import

Electron 3.0.0-beta.1 Node 10.2.0 Chromium 66.0.3359.181

The problem I'm having is importing a module. I created the following protocol:

protocol.registerFileProtocol('client', (request, callback) => {
    var url = request.url.substr(8);
    callback({path: path.join(__dirname, url)});
});

The output of the protocol is the correct path

"/Users/adviner/Projects/Client/src/ClientsApp/app.js"

I have the following module app.js with the following code:

export function square() {
    return 'hello';
}

in my index.html I import the module like so:

    <script type="module" >
        import square from 'client://app.js';
        console.log(square());
    </script>       

But I keep getting the error:

app.js/:1 Failed to load module script: The server responded with a non-JavaScript MIME type of "". Strict MIME type checking is enforced for module scripts per HTML spec.

I'm done searches but can't seem to find a solution. Can anyone suggest a way I can make this work?

Thanks