Move file upload into SMTP send job.

- This adds params for the upload URL and local file path, so that the
actual upload can happen in the send job.
- This also moves the URL generation to the client side so that we can
  generate a valid URL before the upload (because the MIME rendering of
  the mail message happens earlier and we want to include the URL there)

upload-server/package.json

@@ -7,6 +7,7 @@
     "start": "node server.js"
   },
   "dependencies": {
+    "base32": "^0.0.6",
     "express": "^4.17.1"
   }
 }