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？