feat: update reset thumbnail feature (#2854)

* update reset thumbnail feature

* consolidate supported thumbnail extensions into site config; remove dedicated API

* allow patching thumb ; remove Reset Thumbnail API

* fix code formatting

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Co-authored-by: Aaron Liu <abslant.liu@gmail.com>

pkg/thumb/builtin.go

@@ -19,6 +19,10 @@ import (
 
 const thumbTempFolder = "thumb"
 
+// BuiltinSupportedExts lists file extensions supported by the built-in
+// thumbnail generator. Extensions are lowercased and do not include the dot.
+var BuiltinSupportedExts = []string{"jpg", "jpeg", "png", "gif"}
+
 // Thumb 缩略图
 type Thumb struct {
 	src image.Image