手机上使用wx.downloadFile下载oss上的.png图片，返回是.bin格式的文件，到时后续用wx.saveImageToPhotosAlbum保存图片失败，报{errMsg: "saveImageToPhotosAlbum:fail invalid file type"}，但是在开发工具上可以直接返回png格式图片，可以保存。

如下为用wx.downloadFile请求和返回的结果。

         wx.downloadFile({

            url: 'https://prod-auops.oss-cn-shenzhen.aliyuncs.com/wxacode/5/gm.png?Expires=4724616620&OSSAccessKeyId=LW6Q1beRi0R3m14r&Signature=S/v7ibcS1c7nP12EvBsLp3sk3j0%3D',

            success(res) {               

               if (res.statusCode === 200) {

                  console.log(res)

                  wx.saveImageToPhotosAlbum({

                     filePath: res.tempFilePath,

                     success(res) { 

                        console.log(res)

                        wx.showToast({

                           icon: 'none',

                           title: "图片已保存"

                        })

                     },

                     fail(res){

                        console.log(res)

                     }

                  })

               }

            }

         })

返回结果

{tempFilePath: "wxfile://tmp_54d93ed93a853a3a9765337741943c492df65fcd1db83a0a.bin", header: {…}, statusCode: 200, errMsg: "downloadFile:ok"}

errMsg: "downloadFile:ok"

header:

Accept-Ranges: "bytes"

Connection: "keep-alive"

Content-Disposition: "attachment;filename=gm.png"

Content-Length: "103837"

Content-MD5: "T4RW3zvQtapK4rYXVczxJA=="

Content-Type: "application/octet-stream"

Date: "Mon, 14 Oct 2019 01:12:50 GMT"

ETag: ""4F8456DF3BD0B5AA4AE2B61755CCF124""

Last-Modified: "Sun, 13 Oct 2019 14:30:55 GMT"

Server: "AliyunOSS"

X-Android-Received-Millis: "1571015568965"

X-Android-Response-Source: "NETWORK 200"

X-Android-Selected-Protocol: "http/1.1"

X-Android-Sent-Millis: "1571015568954"

x-oss-hash-crc64ecma: "13627780087634526589"

x-oss-object-type: "Normal"

x-oss-request-id: "5DA3CB929ACC073532C38DAE"

x-oss-server-time: "2"

x-oss-storage-class: "Standard"

__proto__: Object

statusCode: 200

tempFilePath: "wxfile://tmp_54d93ed93a853a3a9765337741943c492df65fcd1db83a0a.bin"

__proto__: Object