Fix binary string encoding

The File object used to handle binary strings had an incorrect
receiver for json marshaler. Add a test and fix the problem.

Author: marcinromaszewicz

file.go

@@ -25,7 +25,7 @@ func (file *File) InitFromBytes(data []byte, filename string) {
 	file.multipart = nil
 }
 
-func (file *File) MarshalJSON() ([]byte, error) {
+func (file File) MarshalJSON() ([]byte, error) {
 	b, err := file.Bytes()
 	if err != nil {
 		return nil, err

file_test.go

@@ -0,0 +1,53 @@
+package types
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+var _ json.Marshaler = (*File)(nil)
+var _ json.Unmarshaler = (*File)(nil)
+
+func TestFileJSON(t *testing.T) {
+	type Object struct {
+		BinaryField File `json:"binary_field"`
+	}
+
+	// Check whether we encode JSON properly.
+	var o Object
+	o.BinaryField.InitFromBytes([]byte("hello"), "")
+	buf, err := json.Marshal(o)
+	require.NoError(t, err)
+	t.Log(string(buf))
+
+	// Decode the same object back into File, ensure result is correct.
+	var o2 Object
+	err = json.Unmarshal(buf, &o2)
+	require.NoError(t, err)
+	o2Bytes, err := o2.BinaryField.Bytes()
+	require.NoError(t, err)
+	assert.Equal(t, []byte("hello"), o2Bytes)
+
+	// Ensure it also works via pointer.
+	type Object2 struct {
+		BinaryFieldPtr *File `json:"binary_field"`
+	}
+
+	var o3 Object2
+	var f File
+	f.InitFromBytes([]byte("hello"), "")
+	o3.BinaryFieldPtr = &f
+	buf, err = json.Marshal(o)
+	require.NoError(t, err)
+	t.Log(string(buf))
+
+	var o4 Object2
+	err = json.Unmarshal(buf, &o4)
+	require.NoError(t, err)
+	o4Bytes, err := o4.BinaryFieldPtr.Bytes()
+	assert.Equal(t, []byte("hello"), o4Bytes)
+
+}