Don't generate thumbnails for webp (#2388)

* Don't generate thumbnails for animated webp
* Debug log when writing thumbnail to disk

pkg/image/webp.go

@@ -0,0 +1,46 @@
+package image
+
+import (
+	"bytes"
+)
+
+const (
+	formatWebP = "webp"
+	formatGif  = "gif"
+)
+
+// https://developers.google.com/speed/webp/docs/riff_container
+func isWebPAnimated(buf []byte) bool {
+	const (
+		webPHeaderStart = 8
+		webPHeaderEnd   = 12
+		webPHeader      = "WEBP"
+
+		animationHeaderLoc    = 16
+		minAnimSignatureIndex = 20
+
+		maxSize = 48
+	)
+
+	// truncate the buffer to the max size
+	if len(buf) > maxSize {
+		buf = buf[:maxSize]
+	}
+
+	isWebp := len(buf) >= webPHeaderEnd && string(buf[webPHeaderStart:webPHeaderEnd]) == "WEBP" // is WEBP
+
+	if isWebp {
+		const animBit byte = 1 << 1
+		if len(buf) > minAnimSignatureIndex {
+			// Animation Bit is set and ANIM header is present
+			return (buf[animationHeaderLoc]&animBit == animBit) && containsAnimSignature(buf[minAnimSignatureIndex:])
+		}
+	}
+	return false
+}
+
+// https://developers.google.com/speed/webp/docs/riff_container#animation
+func containsAnimSignature(buf []byte) bool {
+	index := bytes.Index(buf, []byte("ANIM"))
+	return index != -1
+}