uniapp如何把Android根目录的图片转换为base64并上传

                        let environment = plus.android.importClass("android.os.Environment");  
                        var sdRoot = environment.getExternalStorageDirectory()   '/'   res.name;  //文件夹根目录  
                        plus.io.resolveLocalFileSystemURL(sdRoot, function(entry) {  
                          entry.file(function(file) {  
                            var reader = new plus.io.FileReader();  
                            reader.onloadend = function(evt) {  
                               uni.showModal({  
                                 title: 'Base64 字符串',  
                                 content: evt.target.result,  
                                 showCancel: false  
                               });    
                            };  
                            reader.readAsDataURL(file);  
                          }, function(error) {  
                            uni.showModal({  
                              title: '读取文件失败',  
                              content: error.message,  
                              showCancel: false  
                            });  
                          });  
                        }, function(error) {  
                          uni.showModal({  
                            title: '获取文件路径失败',  
                            content: error.message,  
                            showCancel: false  
                          });  
                        });