sequenceDiagram
    participant Browser
    participant Server as Preview Server
    participant EM as EditorManager
    participant FS as FileSystem
    
    Browser->>Server: GET /about (no extension)
    Server->>Server: Extract reqPath = "about"
    Server->>Server: Detect !ext && pathName
    
    Note over Server: Try path.html first
    Server->>EM: getFile("pathName/about.html")
    alt File loaded and unsaved in editor
        EM-->>Server: Return editor file
        Server->>Server: sendHTML(file.session.getValue())
        Server-->>Browser: 200 OK (HTML from editor)
    else Not in editor
        Server->>FS: fsOperation("pathName/about.html").exists()
        alt about.html exists
            FS-->>Server: true
            Server->>Server: sendFileContent(htmlUrl, MIMETYPE_HTML)
            Server-->>Browser: 200 OK (about.html)
        else about.html not found
            FS-->>Server: false
            Note over Server: Try path/index.html fallback
            Server->>FS: fsOperation("pathName/about/index.html").exists()
            alt about/index.html exists
                FS-->>Server: true
                Server->>Server: sendFileContent(indexUrl, MIMETYPE_HTML)
                Server-->>Browser: 200 OK (about/index.html)
            else index.html not found
                FS-->>Server: false
                Server->>Server: error(reqId)
                Server-->>Browser: 404 Not Found
            end
        end
    end