Extend context.File to allow for the content-dispositon attachments via a new method context.Attachment (#1260)

* Add FileAttachment method to context to allow instant downloads with filenames

* Add relevant tests for FileAttachment method

context_test.go

@@ -979,6 +979,19 @@ func TestContextRenderFile(t *testing.T) {
 	assert.Equal(t, "text/plain; charset=utf-8", w.Header().Get("Content-Type"))
 }
 
+func TestContextRenderAttachment(t *testing.T) {
+	w := httptest.NewRecorder()
+	c, _ := CreateTestContext(w)
+	newFilename := "new_filename.go"
+
+	c.Request, _ = http.NewRequest("GET", "/", nil)
+	c.FileAttachment("./gin.go", newFilename)
+
+	assert.Equal(t, 200, w.Code)
+	assert.Contains(t, w.Body.String(), "func New() *Engine {")
+	assert.Equal(t, fmt.Sprintf("attachment; filename=\"%s\"", newFilename), w.HeaderMap.Get("Content-Disposition"))
+}
+
 // TestContextRenderYAML tests that the response is serialized as YAML
 // and Content-Type is set to application/x-yaml
 func TestContextRenderYAML(t *testing.T) {