more about file uploads

Rewritten-docs.md

@@ -1489,12 +1489,17 @@ container implementation.
 
 #### Uploading files
 
-There are two cases when users need to upload files using the API – submitting
+There are two cases when users need to upload files using the API -– submitting
 solutions to an assignment and creating a new exercise. In both of these cases,
-the final destination of the files is the fileserver. 
+the final destination of the files is the fileserver. However, the fileserver is
-
+not publicly accessible, so the files have to be uploaded through the API.
-However, the fileserver is not publicly accessible, so the files have to be
+
-uploaded through the API.
+The files can be either forwarded to the fileserver directly, without any
+interference from the API server, or stored and forwarded later. We chose the
+second approach, which is harder to implement, but more convenient -- it lets
+exercise authors double-check what they upload to the fileserver and solutions
+to assignments can be uploaded in a single request, which makes it easy for the
+fileserver to create an archive of the solution files.
 
 @todo permission handling, roles, etc.