Files

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# Media & Services
## File Upload
Files are uploaded via REST, validated by magic bytes, stored with GUID filenames,
and broadcast as a message with a download link.
```mermaid
sequenceDiagram
participant Client as Client
participant CC as ChannelsController
participant FV as FileValidationHelper
participant FS as FileStorageService
participant CS as ChatService
participant DB as SQLite
Client->>CC: POST /api/channels/{channel}/upload (multipart)
CC->>CC: Check file size limits
CC->>FV: IsValidImage(stream) — magic byte check
FV->>FV: Read header: JPEG(FFD8FF) / PNG(89504E47) / GIF / WebP(RIFF+WEBP)
FV-->>CC: true/false
CC->>FS: SaveFileAsync(stream, extension)
FS->>FS: Generate GUID filename, write to uploads/
FS-->>CC: fileId (GUID)
CC->>CC: Determine MessageType (Image/Audio/File)
CC->>CS: SendMessageAsync (with file URL + optional ASCII art)
CS->>DB: INSERT Message
CS->>CS: BroadcastToAllAsync → fan out to clients
```
---
## Link Embed Resolution
When a message contains URLs, the server fetches OpenGraph metadata for preview
embeds.
```mermaid
sequenceDiagram
participant CS as ChatService
participant LE as LinkEmbedService
participant Web as External Website
CS->>LE: TryGetEmbedsAsync(messageContent)
LE->>LE: Extract URLs via regex
LE->>LE: Filter: max URLs per message, skip duplicates
loop For each URL
LE->>LE: Validate: http(s) only, block private IPs
LE->>Web: GET URL (timeout + size limit)
Web-->>LE: HTML response
LE->>LE: Parse og:title, og:description, og:site_name
LE->>LE: Parse theme-color meta tag
LE->>LE: Fallback to <title> if no og:title
end
LE-->>CS: List<EmbedDto> (or null)
Note over CS: Attached to MessageDto before broadcast
```
---
## Server Directory Registration
Public servers register with the EchoHubSpace directory for discoverability.
```mermaid
sequenceDiagram
participant SDS as ServerDirectoryService
participant Dir as EchoHubSpace Directory
participant PT as PresenceTracker
SDS->>SDS: Check Server:PublicServer config
SDS->>Dir: SignalR connect (echohub.voidcube.cloud/hubs/servers)
SDS->>Dir: RegisterServer(name, description, host, userCount)
Dir-->>SDS: Registered
loop Every 30 seconds
SDS->>PT: Get online user count
alt Count changed
SDS->>Dir: UpdateUserCount(count)
end
end
Dir->>SDS: Ping
SDS->>Dir: Heartbeat
Note over SDS,Dir: Exponential backoff on disconnect (2s → 30s max)
```