Can not read file content from 1C web app hosted in the cloud

I am trying to read content of text file selected from local computer from my 1C web app hosted in the cloud using methods FileStreams.OpenForRead(Filename) or New BinaryData(Filename) and I get an error message "[ScriptRuntimeError, ScriptUseError], reason: Directory not found". What can be the reason?
If I launch my web app from localhost everything works fine.

Hey Peter,

let me some time pls - I'll create a small example how to work with files in web client in 1C

I solved already this problem. Here's the solution:
Stream = Await FileStreams.OpenForReadAsync( Filename );
FileReader = New DataReader( Stream, TextEncoding.UTF8 );  
FileContent = Await FileReader.ReadCharsAsync( , TextEncoding.UTF8 );