Document why key comes first

js/modules/debuglogs.js

@@ -26,8 +26,8 @@ exports.upload = async (content) => {
   const { fields, url } = signedForm.body;
 
   const form = new FormData();
+  // The API expects `key` to be the first field:
   form.append('key', fields.key);
-
   Object.entries(fields)
     .filter(([key]) => key !== 'key')
     .forEach(([key, value]) => {