Add content detector

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

api/layer/detector.go

@@ -0,0 +1,26 @@
+package layer
+
+import (
+	"io"
+	"net/http"
+	"sync"
+)
+
+type detector struct {
+	io.Reader
+	sync.Once
+
+	contentType string
+}
+
+func newDetector(r io.Reader) *detector {
+	return &detector{Reader: r}
+}
+
+func (d *detector) Read(data []byte) (int, error) {
+	d.Do(func() {
+		d.contentType = http.DetectContentType(data)
+	})
+
+	return d.Reader.Read(data)
+}