fileserver analysis introduction

Rewritten-docs.md

@@ -1195,7 +1195,14 @@ term project for C# course so it might be written and integrated in future.
 
 ### Fileserver
 
-@todo: fileserver and why is separated
+The fileserver provides access to a shared storage space that contains files
+submitted by students, supplementary files such as test inputs and outputs and
+results of evaluation. This functionality can be easily separated from the rest
+of the backend features, which led to designing the fileserver as a
+standalone component. Such design helps encapsulate the details of how the files
+are stored (e.g. on a file system, in a database or using a cloud storage
+service), while also making it possible to share the storage between multiple
+ReCodEx frontends.
 
 @todo: mention hashing on fileserver and why this approach was chosen