a bit about file uploads

Rewritten-docs.md

@@ -1489,6 +1489,15 @@ container implementation.
 
 @todo: what files are stored in api, why there are duplicates among api and fileserver
 
+#### Uploading files
+
+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. 
+
+However, the fileserver is not publicly accessible, so the files have to be
+uploaded through the API.
+
 @todo: groups and hierarchy, describe arbitrary nesting which should be possible
 within instance and how it is implemented and how it could be implemented
 (describe only implementation if it is something what to say)