feat: Add support for message attachments

- Introduced Attachment model to handle file attachments associated with messages.
- Updated ModerationController to manage message deletions and attachment cleanup.
- Enhanced ChatService to include attachments in message retrieval.
- Implemented migration for legacy single-attachment messages to the new Attachments model.
- Added unit tests for attachment handling in message formatting and parsing.
- Updated database context and migrations to support new Attachments table.

src/EchoHub.Core/Constants/HubConstants.cs

@@ -11,6 +11,7 @@ public static class HubConstants
     public const int MaxFileSizeBytes = 100 * 1024 * 1024; // 100 MB
     public const int MaxAvatarSizeBytes = 2 * 1024 * 1024; // 2 MB
     public const int MaxMessageNewlines = 30;
+    public const int MaxAttachmentsPerMessage = 10;
     public const int MaxConsecutiveNewlines = 1;
     public const int AsciiArtWidth = 80;
     public const int AsciiArtHeight = 40;

src/EchoHub.Core/DTOs/ChatDtos.cs

@@ -8,13 +8,22 @@ public record MessageDto(
     string SenderUsername,
     string? SenderNicknameColor,
     string ChannelName,
-    MessageType Type,
-    string? AttachmentUrl,
-    string? AttachmentFileName,
     DateTimeOffset SentAt,
-    long? AttachmentFileSize = null,
+    List<AttachmentDto>? Attachments = null,
     List<EmbedDto>? Embeds = null);
 
+/// <summary>
+/// A file attached to a message. <see cref="AsciiPreview"/> holds the color-tag art for
+/// images (null otherwise). For end-to-end encrypted channels the content behind
+/// <see cref="Url"/> and the preview are ciphertext the server cannot read.
+/// </summary>
+public record AttachmentDto(
+    AttachmentKind Kind,
+    string Url,
+    string FileName,
+    long FileSize,
+    string? AsciiPreview = null);
+
 public record ChannelDto(
     Guid Id,
     string Name,

src/EchoHub.Core/Models/Attachment.cs

@@ -0,0 +1,28 @@
+namespace EchoHub.Core.Models;
+
+/// <summary>
+/// A file attached to a message (image, audio, or any file). A message may carry
+/// zero or more attachments alongside its text content (Discord-style).
+/// </summary>
+public class Attachment
+{
+    public Guid Id { get; set; }
+    public Guid MessageId { get; set; }
+    public Message? Message { get; set; }
+
+    public AttachmentKind Kind { get; set; }
+
+    /// <summary>Relative download URL, e.g. <c>/api/files/{fileId}</c>.</summary>
+    public required string Url { get; set; }
+    public required string FileName { get; set; }
+
+    /// <summary>Stored blob size in bytes (ciphertext size for encrypted channels).</summary>
+    public long FileSize { get; set; }
+
+    /// <summary>
+    /// Rendered ASCII-art preview for images (color-tag format). Null for audio/files.
+    /// Stored encrypted-at-rest when database encryption is enabled, and room-encrypted
+    /// for end-to-end encrypted channels.
+    /// </summary>
+    public string? AsciiPreview { get; set; }
+}

src/EchoHub.Core/Models/AttachmentKind.cs

@@ -0,0 +1,12 @@
+namespace EchoHub.Core.Models;
+
+/// <summary>
+/// The kind of a message attachment. Determines how the client renders it
+/// (ASCII preview for images, a play affordance for audio, a download line for files).
+/// </summary>
+public enum AttachmentKind
+{
+    Image,
+    Audio,
+    File
+}

src/EchoHub.Core/Models/Message.cs

@@ -3,11 +3,10 @@ namespace EchoHub.Core.Models;
 public class Message
 {
     public Guid Id { get; set; }
+
+    /// <summary>The message text/caption. May be empty when the message only carries attachments.</summary>
     public required string Content { get; set; }
-    public MessageType Type { get; set; } = MessageType.Text;
-    public string? AttachmentUrl { get; set; }
-    public string? AttachmentFileName { get; set; }
-    public long? AttachmentFileSize { get; set; }
+
     public string? EmbedJson { get; set; }
     public DateTimeOffset SentAt { get; set; } = DateTimeOffset.UtcNow;
 
@@ -16,4 +15,16 @@ public class Message
 
     public Guid SenderUserId { get; set; }
     public required string SenderUsername { get; set; }
+
+    /// <summary>Files attached to this message. Empty for a plain text message.</summary>
+    public List<Attachment> Attachments { get; set; } = [];
+
+    // ── Legacy columns (pre-attachments model) ──────────────────────────────
+    // Retained so the one-time startup data migration can fold old single-attachment
+    // messages into Attachments. New code never writes these; they are nulled out
+    // once migrated. Not exposed in DTOs. See DataMigrationService.MigrateLegacyAttachmentsAsync.
+    public MessageType Type { get; set; } = MessageType.Text;
+    public string? AttachmentUrl { get; set; }
+    public string? AttachmentFileName { get; set; }
+    public long? AttachmentFileSize { get; set; }
 }