Refactored NSData ImageContentType category, instead of returning the contentTyp…

…e as a string, the new added method `sd_imageFormatForImageData` will return a `SDImageFormat` enum value

@@ -79,8 +79,8 @@ static char imageURLKey;
                     completedBlock(image, error, cacheType, url);
                     return;
                 } else if (image) {
-                    NSString *imageContentType = [NSData sd_contentTypeForImageData:data];
-                    if ([imageContentType isEqualToString:@"image/gif"]) {
+                    SDImageFormat imageFormat = [NSData sd_imageFormatForImageData:data];
+                    if (imageFormat == SDImageFormatGIF) {
                         wself.animatedImage = [FLAnimatedImage animatedImageWithGIFData:data];
                         wself.image = nil;
                     } else {

@@ -9,15 +9,24 @@
 
 #import <Foundation/Foundation.h>
 
+typedef NS_ENUM(NSInteger, SDImageFormat) {
+    SDImageFormatUndefined = -1,
+    SDImageFormatJPEG = 0,
+    SDImageFormatPNG,
+    SDImageFormatGIF,
+    SDImageFormatTIFF,
+    SDImageFormatWebP
+};
+
 @interface NSData (ImageContentType)
 
 /**
- *  Compute the content type for an image data
+ *  Return image format
  *
- *  @param data the input data
+ *  @param data the input image data
  *
- *  @return the content type as string (i.e. image/jpeg, image/gif)
+ *  @return the image format as `SDImageFormat` (enum)
  */
-+ (nullable NSString *)sd_contentTypeForImageData:(nullable NSData *)data;
++ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data;
 
 @end

@@ -12,37 +12,35 @@
 
 @implementation NSData (ImageContentType)
 
-+ (nullable NSString *)sd_contentTypeForImageData:(nullable NSData *)data {
++ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data {
     if (!data) {
-        return nil;
+        return SDImageFormatUndefined;
     }
-
+    
     uint8_t c;
     [data getBytes:&c length:1];
     switch (c) {
         case 0xFF:
-            return @"image/jpeg";
+            return SDImageFormatJPEG;
         case 0x89:
-            return @"image/png";
+            return SDImageFormatPNG;
         case 0x47:
-            return @"image/gif";
+            return SDImageFormatGIF;
         case 0x49:
         case 0x4D:
-            return @"image/tiff";
+            return SDImageFormatTIFF;
         case 0x52:
             // R as RIFF for WEBP
             if (data.length < 12) {
-                return nil;
+                return SDImageFormatUndefined;
             }
-
+            
             NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
             if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
-                return @"image/webp";
+                return SDImageFormatWebP;
             }
-
-            return nil;
     }
-    return nil;
+    return SDImageFormatUndefined;
 }
 
 @end

@@ -23,12 +23,12 @@
     }
     
     UIImage *image;
-    NSString *imageContentType = [NSData sd_contentTypeForImageData:data];
-    if ([imageContentType isEqualToString:@"image/gif"]) {
+    SDImageFormat imageFormat = [NSData sd_imageFormatForImageData:data];
+    if (imageFormat == SDImageFormatGIF) {
         image = [UIImage sd_animatedGIFWithData:data];
     }
 #ifdef SD_WEBP
-    else if ([imageContentType isEqualToString:@"image/webp"])
+    else if (imageFormat == SDImageFormatWebP)
     {
         image = [UIImage sd_imageWithWebPData:data];
     }