fix file URL validator (#1171)

## Fixes Or Enhances

This PR adds an additional check for the file URL schema.
Connected to #1165

**Make sure that you've checked the boxes below before you submit PR:**
- [x] Tests exist or have been written that cover this particular
change.

@go-playground/validator-maintainers

Author: nodivbyzero

baked_in.go

@@ -1414,19 +1414,32 @@ func isURI(fl FieldLevel) bool {
 	panic(fmt.Sprintf("Bad field type %T", field.Interface()))
 }
 
+// isFileURL is the helper function for validating if the `path` valid file URL as per RFC8089
+func isFileURL(path string) bool {
+	if !strings.HasPrefix(path, "file:/") {
+		return false
+	}
+	_, err := url.ParseRequestURI(path)
+	return err == nil
+}
+
 // isURL is the validation function for validating if the current field's value is a valid URL.
 func isURL(fl FieldLevel) bool {
 	field := fl.Field()
 
 	switch field.Kind() {
 	case reflect.String:
 
-		s := field.String()
+		s := strings.ToLower(field.String())
 
 		if len(s) == 0 {
 			return false
 		}
 
+		if isFileURL(s) {
+			return true
+		}
+
 		url, err := url.Parse(s)
 		if err != nil || url.Scheme == "" {
 			return false

validator_test.go

@@ -8139,6 +8139,12 @@ func TestUrl(t *testing.T) {
 		{"./rel/test/dir", false},
 		{"irc:", false},
 		{"http://", false},
+		{"file://path/to/file.txt", true},
+		{"file:///c:/Windows/file.txt", true},
+		{"file://localhost/path/to/file.txt", true},
+		{"file://localhost/c:/WINDOWS/file.txt", true},
+		{"file://", true},
+		{"file:////remotehost/path/file.txt", true},
 	}
 
 	validate := New()