diff --git a/SDWebImage/NSData+ImageContentType.h b/SDWebImage/NSData+ImageContentType.h
index 2a9f7e2b..c5970cb4 100644
--- a/SDWebImage/NSData+ImageContentType.h
+++ b/SDWebImage/NSData+ImageContentType.h
@@ -10,15 +10,18 @@
 #import <Foundation/Foundation.h>
 #import "SDWebImageCompat.h"
 
-typedef NS_ENUM(NSInteger, SDImageFormat) {
-    SDImageFormatUndefined = -1,
-    SDImageFormatJPEG = 0,
-    SDImageFormatPNG,
-    SDImageFormatGIF,
-    SDImageFormatTIFF,
-    SDImageFormatWebP,
-    SDImageFormatHEIC
-};
+/**
+ You can use switch case like normal enum. It's also recommended to add a default case. You should not assume anything about the raw value.
+ For custom coder plugin, it can also extern the enum for supported format. See `SDImageCoder` for more detailed information.
+ */
+typedef NSInteger SDImageFormat NS_TYPED_EXTENSIBLE_ENUM;
+static const SDImageFormat SDImageFormatUndefined = -1;
+static const SDImageFormat SDImageFormatJPEG      = 0;
+static const SDImageFormat SDImageFormatPNG       = 1;
+static const SDImageFormat SDImageFormatGIF       = 2;
+static const SDImageFormat SDImageFormatTIFF      = 3;
+static const SDImageFormat SDImageFormatWebP      = 4;
+static const SDImageFormat SDImageFormatHEIC      = 5;
 
 @interface NSData (ImageContentType)
 
diff --git a/SDWebImage/SDImageCoder.h b/SDWebImage/SDImageCoder.h
index ebf151b2..d6c7a735 100644
--- a/SDWebImage/SDImageCoder.h
+++ b/SDWebImage/SDImageCoder.h
@@ -72,6 +72,10 @@ FOUNDATION_EXPORT SDImageCoderOption _Nonnull const SDImageCoderEncodeCompressio
 
 /**
  Returns YES if this coder can encode some image. Otherwise, it should be passed to another coder.
+ For custom coder which introduce new image format, you'd better define a new `SDImageFormat` using like this. If you're creating public coder plugin for new image format, also update `https://github.com/rs/SDWebImage/wiki/Coder-Plugin-List` to avoid same value been defined twice.
+ * @code
+ static const SDImageFormat SDImageFormatHEIF = 10;
+ * @endcode
  
  @param format The image format
  @return YES if this coder can encode the image, NO otherwise