Allow leading period in FindFormatByFileExtension

src/ImageSharp/Formats/ImageFormatManager.cs

@@ -85,6 +85,13 @@ namespace SixLabors.ImageSharp.Formats
         /// <returns>The <see cref="IImageFormat"/> if found otherwise null</returns>
         public IImageFormat FindFormatByFileExtension(string extension)
         {
+            Guard.NotNullOrWhiteSpace(extension, nameof(extension));
+
+            if (extension[0] == '.')
+            {
+                extension = extension.Substring(1);
+            }
+
             return this.imageFormats.FirstOrDefault(x => x.FileExtensions.Contains(extension, StringComparer.OrdinalIgnoreCase));
         }
 

src/ImageSharp/ImageExtensions.cs

@@ -32,7 +32,7 @@ namespace SixLabors.ImageSharp
         {
             Guard.NotNullOrWhiteSpace(filePath, nameof(filePath));
 
-            string ext = Path.GetExtension(filePath).Trim('.');
+            string ext = Path.GetExtension(filePath);
             IImageFormat format = source.GetConfiguration().ImageFormatsManager.FindFormatByFileExtension(ext);
             if (format == null)
             {

tests/ImageSharp.Tests/TestUtilities/TestEnvironment.Formats.cs

@@ -32,8 +32,8 @@ namespace SixLabors.ImageSharp.Tests
 
         internal static IImageFormat GetImageFormat(string filePath)
         {
-            string extension = Path.GetExtension(filePath).ToLower();
-            if (extension[0] == '.') extension = extension.Substring(1);
+            string extension = Path.GetExtension(filePath);
+            
             IImageFormat format = Configuration.ImageFormatsManager.FindFormatByFileExtension(extension);
             return format;
         }