wx.downloadFile({

        url:url1,

        header: {'content-type':'image/png'},

        success: (result)=>{

          console.log(result)

          wx.saveImageToPhotosAlbum({

            filePath: result.tempFilePath,

            success: (res)=>{

              console.log('保存成功')

            },

            fail: ()=>{},

            complete: ()=>{}

          });

        },

      });

    } 

文件名后缀总是bin,请问为什么？