Retain underlying reader if it's an io.Seeker

formats.go

@@ -319,7 +319,14 @@ func (rr *rewindReader) rewind() {
 // bytes, then from the underlying stream. After calling this,
 // no more rewinding is allowed since reads from the stream are
 // not recorded, so rewinding properly is impossible.
+// If the underlying reader implements io.Seeker, then the
+// underlying reader will be used directly.
 func (rr *rewindReader) reader() io.Reader {
+	if ras, ok := rr.Reader.(io.Seeker); ok {
+		if _, err := ras.Seek(-int64(rr.buf.Len()), io.SeekCurrent); err == nil {
+			return rr.Reader
+		}
+	}
 	return io.MultiReader(bytes.NewReader(rr.buf.Bytes()), rr.Reader)
 }
 

formats_test.go

@@ -387,3 +387,26 @@ func TestIdentifyFindFormatByStreamContent(t *testing.T) {
 		})
 	}
 }
+
+func TestIdentifyAndOpenZip(t *testing.T) {
+	f, err := os.Open("testdata/test.zip")
+	checkErr(t, err, "opening zip")
+	defer f.Close()
+
+	format, reader, err := Identify("test.zip", f)
+	checkErr(t, err, "identifying zip")
+	if format.Name() != ".zip" {
+		t.Fatalf("unexpected format found: expected=.zip actual:%s", format.Name())
+	}
+
+	err = format.(Extractor).Extract(context.Background(), reader, nil, func(ctx context.Context, f File) error {
+		rc, err := f.Open()
+		if err != nil {
+			return err
+		}
+		defer rc.Close()
+		_, err = io.ReadAll(rc)
+		return err
+	})
+	checkErr(t, err, "extracting zip")
+}