Added the missing extension `apng` for YYImage

YYWebImage/Image/YYImage.m

@@ -102,7 +102,7 @@ static CGFloat _NSStringPathScale(NSString *string) {
     CGFloat scale = 1;
     
     // If no extension, guess by system supported (same as UIImage).
-    NSArray *exts = ext.length > 0 ? @[ext] : @[@"", @"png", @"jpeg", @"jpg", @"gif", @"webp"];
+    NSArray *exts = ext.length > 0 ? @[ext] : @[@"", @"png", @"jpeg", @"jpg", @"gif", @"webp", @"apng"];
     NSArray *scales = _NSBundlePreferredScales();
     for (int s = 0; s < scales.count; s++) {
         scale = ((NSNumber *)scales[s]).floatValue;