Made the default IImageFormatDetectors public and sealed.

src/ImageSharp/Formats/Bmp/BmpImageFormatDetector.cs

@@ -10,7 +10,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Detects bmp file headers
     /// </summary>
-    internal class BmpImageFormatDetector : IImageFormatDetector
+    public sealed class BmpImageFormatDetector : IImageFormatDetector
     {
         /// <inheritdoc/>
         public int HeaderSize => 2;

src/ImageSharp/Formats/Gif/GifImageFormatDetector.cs

@@ -10,7 +10,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Detects gif file headers
     /// </summary>
-    public class GifImageFormatDetector : IImageFormatDetector
+    public sealed class GifImageFormatDetector : IImageFormatDetector
     {
         /// <inheritdoc/>
         public int HeaderSize => 6;

src/ImageSharp/Formats/Jpeg/JpegImageFormatDetector.cs

@@ -10,7 +10,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Detects Jpeg file headers
     /// </summary>
-    public class JpegImageFormatDetector : IImageFormatDetector
+    public sealed class JpegImageFormatDetector : IImageFormatDetector
     {
         /// <inheritdoc/>
         public int HeaderSize => 11;

src/ImageSharp/Formats/Png/PngImageFormatDetector.cs

@@ -10,7 +10,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Detects png file headers
     /// </summary>
-    public class PngImageFormatDetector : IImageFormatDetector
+    public sealed class PngImageFormatDetector : IImageFormatDetector
     {
         /// <inheritdoc/>
         public int HeaderSize => 8;