Add new FileOutput type to stream.js

This takes a url and creates a `ReadableStream` that has two additional
methods `url()` and `blob()` for easily working with remote URLs or
base64 encoded data.

Author: aron

lib/stream.js

@@ -98,7 +98,61 @@ function createReadableStream({ url, fetch, options = {} }) {
   });
 }
 
+function createFileOutput({ url, fetch }) {
+  let type = "application/octet-stream";
+
+  class FileOutput extends ReadableStream {
+    async blob() {
+      const chunks = [];
+      for await (const chunk of this) {
+        chunks.push(chunk);
+      }
+      return new Blob(chunks, { type });
+    }
+
+    url() {
+      return new URL(url);
+    }
+
+    toString() {
+      return url;
+    }
+  }
+
+  return new FileOutput({
+    async start(controller) {
+      const response = await fetch(url);
+
+      if (!response.ok) {
+        const text = await response.text();
+        const request = new Request(url, init);
+        controller.error(
+          new ApiError(
+            `Request to ${url} failed with status ${response.status}: ${text}`,
+            request,
+            response
+          )
+        );
+      }
+
+      if (response.headers.get("Content-Type")) {
+        type = response.headers.get("Content-Type");
+      }
+
+      try {
+        for await (const chunk of streamAsyncIterator(response.body)) {
+          controller.enqueue(chunk);
+        }
+        controller.close();
+      } catch (err) {
+        controller.error(err);
+      }
+    },
+  });
+}
+
 module.exports = {
+  createFileOutput,
   createReadableStream,
   ServerSentEvent,
 };