[HTML] Special case plain image embeds

Closes #537

DiscordChatExporter.Core/Utils/FileFormat.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+
+namespace DiscordChatExporter.Core.Utils
+{
+    public static class FileFormat
+    {
+        private static readonly HashSet<string> ImageFormats = new(StringComparer.OrdinalIgnoreCase)
+        {
+            ".jpg",
+            ".jpeg",
+            ".png",
+            ".gif",
+            ".gifv",
+            ".bmp",
+            ".webp"
+        };
+
+        public static bool IsImage(string format) => ImageFormats.Contains(format);
+
+        private static readonly HashSet<string> VideoFormats = new(StringComparer.OrdinalIgnoreCase)
+        {
+            ".mp4",
+            ".webm",
+            ".mov"
+        };
+
+        public static bool IsVideo(string format) => VideoFormats.Contains(format);
+
+        private static readonly HashSet<string> AudioFormats = new(StringComparer.OrdinalIgnoreCase)
+        {
+            ".mp3",
+            ".wav",
+            ".ogg",
+            ".flac",
+            ".m4a"
+        };
+
+        public static bool IsAudio(string format) => AudioFormats.Contains(format);
+    }
+}