feat: enhance message decryption for IRC clients and improve attachment handling

src/EchoHub.Tests/Irc/IrcBroadcasterTests.cs

@@ -89,6 +89,27 @@ public class IrcBroadcasterTests
         Assert.DoesNotContain(output, l => l.Contains("$ENC$"));
     }
 
+    [Fact]
+    public async Task SendMessage_DecryptsAttachmentAsciiPreview()
+    {
+        var (_, stream) = AddConnectionWithCapture("bob", "general");
+
+        var attachment = new AttachmentDto(
+            AttachmentKind.Image, "/api/files/abc", "photo.png", 1234,
+            _encryption.Encrypt("line1\nline2"));
+        var message = new MessageDto(
+            Guid.NewGuid(), _encryption.Encrypt("look at this"), "alice", null, "general",
+            DateTimeOffset.UtcNow, [attachment]);
+
+        await _broadcaster.SendMessageToChannelAsync("general", message);
+
+        var output = stream.GetOutputLines();
+        Assert.Contains(output, l => l.Contains("[Image: photo.png]"));
+        Assert.Contains(output, l => l.Contains("line1"));
+        Assert.Contains(output, l => l.Contains("line2"));
+        Assert.DoesNotContain(output, l => l.Contains("$ENC$"));
+    }
+
     [Fact]
     public async Task SendMessage_SkipsSender()
     {

src/EchoHub.Tests/Irc/IrcMessageFormatterTests.cs

@@ -138,6 +138,19 @@ public class IrcMessageFormatterTests
         Assert.Equal(2, asciiLines.Count);
     }
 
+    [Theory]
+    [InlineData("$ENC$v1$abc123$def456")] // transport ciphertext a broadcast path forgot to decrypt
+    [InlineData("$RC1$abc123def456")]     // E2E room ciphertext the server cannot decrypt
+    public void FormatMessage_ImageMessage_SkipsCiphertextPreview(string ciphertextPreview)
+    {
+        var msg = CreateImageMessage(ciphertextPreview, "photo.jpg", "https://example.com/photo.jpg");
+        var lines = IrcMessageFormatter.FormatMessage(msg);
+
+        // Only the [Image: ...] header line — never the ciphertext blob
+        Assert.Single(lines);
+        Assert.Contains("[Image: photo.jpg]", lines[0]);
+    }
+
     [Fact]
     public void FormatMessage_FileMessage_FormatsCorrectly()
     {