Fix video attachments getting rendered as images

Fixes #120

DiscordChatExporter.Core/Models/Attachment.cs

@@ -1,4 +1,5 @@
-using ByteSizeLib;
+using System;
+using ByteSizeLib;
 
 namespace DiscordChatExporter.Core.Models
 {
@@ -14,10 +15,14 @@ namespace DiscordChatExporter.Core.Models
 
         public int? Height { get; }
 
-        public bool IsImage => Width != null;
-
         public string FileName { get; }
 
+        public bool IsImage => FileName.EndsWith(".jpg", StringComparison.OrdinalIgnoreCase) ||
+                               FileName.EndsWith(".jpeg", StringComparison.OrdinalIgnoreCase) ||
+                               FileName.EndsWith(".png", StringComparison.OrdinalIgnoreCase) ||
+                               FileName.EndsWith(".gif", StringComparison.OrdinalIgnoreCase) ||
+                               FileName.EndsWith(".bmp", StringComparison.OrdinalIgnoreCase);
+
         public ByteSize FileSize { get; }
 
         public Attachment(string id, int? width, int? height, string url, string fileName, ByteSize fileSize)