Add duplex parameter to a streaming request

Attempt to solve the following error in the browser's console when trying to
make the request:

    [error] download and save failed: TypeError: Failed to construct 'Request':
    The `duplex` member must be specified for a request with a streaming body

web/apps/photos/src/services/export/index.ts

@@ -996,10 +996,26 @@ class ExportService {
                     // TODO(MR): Productionalize
                     if (isDevBuild) {
                         console.log({ a: "will send req", updatedFileStream });
+                        // The duplex parameter needs to be set to 'half' when
+                        // streaming requests.
+                        //
+                        // Currently browsers, and specifically in our case,
+                        // since this code runs only within our desktop
+                        // (Electron) app, Chromium, don't support 'full' duplex
+                        // mode (i.e. streaming both the request and the
+                        // response).
+                        //
+                        // https://developer.chrome.com/docs/capabilities/web-apis/fetch-streaming-requests
+                        //
+                        // In another twist, the TypeScript libdom.d.ts does not
+                        // include the "duplex" parameter, so we need to cast to
+                        // get TypeScript to let this code through. e.g. see
+                        // https://github.com/node-fetch/node-fetch/issues/1769
                         const req = new Request("stream://foo", {
                             method: "POST",
                             body: updatedFileStream,
-                        });
+                            duplex: "half",
+                        } as unknown as RequestInit);
                         const res = await fetch(req);
                         console.log({ a: "got res", res });
                     }