fix(MultipartBodyWithJSON): form file field name format (bwmarrin#1256)

Author: FedorLap2006

util.go

@@ -51,7 +51,7 @@ func MultipartBodyWithJSON(data interface{}, files []*File) (requestContentType
 
 	for i, file := range files {
 		h := make(textproto.MIMEHeader)
-		h.Set("Content-Disposition", fmt.Sprintf(`form-data; name="file%d"; filename="%s"`, i, quoteEscaper.Replace(file.Name)))
+		h.Set("Content-Disposition", fmt.Sprintf(`form-data; name="files[%d]"; filename="%s"`, i, quoteEscaper.Replace(file.Name)))
 		contentType := file.ContentType
 		if contentType == "" {
 			contentType = "application/octet-stream"