added Test for decoding windows BMPv2 and one for a bitmap which has 4 bytes per color palette

tests/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs

@@ -55,6 +55,30 @@ namespace SixLabors.ImageSharp.Tests
             }
         }
 
+        [Theory]
+        [WithFile(WinBmpv2, PixelTypes.Rgba32)]
+        public void BmpDecoder_CanDecodeBmpv2<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
+            {
+                image.DebugSave(provider, "png");
+                image.CompareToOriginal(provider);
+            }
+        }
+
+        [Theory]
+        [WithFile(Bit8Palette4, PixelTypes.Rgba32)]
+        public void BmpDecoder_CanDecode4BytePerEntryPalette<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new BmpDecoder()))
+            {
+                image.DebugSave(provider, "png");
+                image.CompareToOriginal(provider);
+            }
+        }
+
         [Theory]
         [InlineData(Car, 24)]
         [InlineData(F, 24)]

tests/ImageSharp.Tests/TestImages.cs

@@ -201,6 +201,9 @@ namespace SixLabors.ImageSharp.Tests
             public const string Bit16 = "Bmp/test16.bmp";
             public const string Bit16Inverted = "Bmp/test16-inverted.bmp";
             public const string Bit32Rgb = "Bmp/rgb32.bmp";
+            // Note: This format can be called OS/2 BMPv1, or Windows BMPv2
+            public const string WinBmpv2 = "Bmp/pal8os2v1_winv2.bmp";
+            public const string Bit8Palette4 = "Bmp/pal8-0.bmp";
 
             public static readonly string[] All
             = {

tests/Images/Input/Bmp/pal8-0.bmp

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:5ae0f808fea31352b8667fdaf897d057c7e02483c43d43cd735f92f4f149fe8d
+size 9270

tests/Images/Input/Bmp/pal8os2v1_winv2.bmp

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:2bfe739377020722872f7fa3ba3ff3ff0daa17f2f202c80e25cf66c4ec30e506
+size 8986