fix: minor fixes after review

mp4/file.go

@@ -331,9 +331,10 @@ func (f *File) findAndReadMfra(r io.Reader) error {
 	if !ok {
 		return fmt.Errorf("expecting readseeker when decoding file ISM file")
 	}
-	pos, err := rs.Seek(-16, io.SeekEnd)
+	mfroSize := int64(16) // This is the fixed size of the mfro box
+	pos, err := rs.Seek(-mfroSize, io.SeekEnd)
 	if err != nil {
-		return fmt.Errorf("could not seek 32 bytes from end: %w", err)
+		return fmt.Errorf("could not seek %d bytes from end: %w", mfroSize, err)
 	}
 	b, err := DecodeBox(uint64(pos), rs) // mfro
 	if err != nil {
@@ -350,11 +351,11 @@ func (f *File) findAndReadMfra(r io.Reader) error {
 	}
 	b, err = DecodeBox(uint64(pos), rs) // mfra
 	if err != nil {
-		return fmt.Errorf("could not decode mfro box: %w", err)
+		return fmt.Errorf("could not decode mfra box: %w", err)
 	}
 	mfra, ok := b.(*MfraBox)
 	if !ok {
-		return fmt.Errorf("expecting mfro box, but got %T", b)
+		return fmt.Errorf("expecting mfra box, but got %T", b)
 	}
 	if len(mfra.Tfras) != 1 {
 		return fmt.Errorf("only supports exactly one tfra in mfra")