refactor(form_mapping.go): mapping multipart request (#1829)

* refactor(form_mapping.go): mapping multipart request

* add checkers for a types to match with the setter interface

* form_mapping.go: rename method name on setter interface, add comments

* fix style of comments

binding/form.go

@@ -4,7 +4,11 @@
 
 package binding
 
-import "net/http"
+import (
+	"mime/multipart"
+	"net/http"
+	"reflect"
+)
 
 const defaultMemory = 32 * 1024 * 1024
 
@@ -53,13 +57,33 @@ func (formMultipartBinding) Bind(req *http.Request, obj interface{}) error {
 	if err := req.ParseMultipartForm(defaultMemory); err != nil {
 		return err
 	}
-	if err := mapForm(obj, req.MultipartForm.Value); err != nil {
-		return err
-	}
-
-	if err := mapFiles(obj, req); err != nil {
+	if err := mappingByPtr(obj, (*multipartRequest)(req), "form"); err != nil {
 		return err
 	}
 
 	return validate(obj)
 }
+
+type multipartRequest http.Request
+
+var _ setter = (*multipartRequest)(nil)
+
+var (
+	multipartFileHeaderStructType = reflect.TypeOf(multipart.FileHeader{})
+)
+
+// TrySet tries to set a value by the multipart request with the binding a form file
+func (r *multipartRequest) TrySet(value reflect.Value, field reflect.StructField, key string, opt setOptions) (isSetted bool, err error) {
+	if value.Type() == multipartFileHeaderStructType {
+		_, file, err := (*http.Request)(r).FormFile(key)
+		if err != nil {
+			return false, err
+		}
+		if file != nil {
+			value.Set(reflect.ValueOf(*file))
+			return true, nil
+		}
+	}
+
+	return setByForm(value, field, r.MultipartForm.Value, key, opt)
+}