核心代码：

index.wxml

  <mp-uploader bindfail="uploadError" bindsuccess="uploadSuccess" select="{{selectFile}}" upload="{{uplaodFile}}" files="{{files}}" max-count="5" title="图片上传" tips="图片上传提示"></mp-uploader>
           

index.js

chooseImage: function (e) {
        var that = this;
        wx.chooseMedia({
            sizeType: ['original', 'compressed'], // 可以指定是原图还是压缩图，默认二者都有
            sourceType: ['album', 'camera'], // 可以指定来源是相册还是相机，默认二者都有
            success: function (res) {
                // 返回选定照片的本地文件路径列表，tempFilePath可以作为img标签的src属性显示图片
                that.setData({
                    files: that.data.files.concat(res.tempFilePaths)
                });
            }
        })
    },
    previewImage: function(e){
        wx.previewImage({
            current: e.currentTarget.id, // 当前显示图片的http链接
            urls: this.data.files // 需要预览的图片http链接列表
        })
    },
    selectFile(files) {
        console.log('files', files)
        // 返回false可以阻止某次文件上传
    },
    uplaodFile(files) {
        let arrayBuffer = files.contents[0];
        let str = '';
        const chunk = 8 * 1024;
        let i;
        for (i = 0; i < arrayBuffer.byteLength / chunk; i++) {
          str += String.fromCharCode(...new Uint8Array(arrayBuffer.slice(i * chunk, (i + 1) * chunk)));
        }
        str += String.fromCharCode(...new Uint8Array(arrayBuffer.slice(i * chunk)));
        let base64 = `data:image/jpeg;base64,${btoa(str)}`;
         
        // 文件上传的函数，返回一个promise
        return new Promise((resolve, reject) => {
           
            wx.request({
                url: 'http://xxxxxx/insertPic',
                method: 'POST',
                data: {
                    pic: base64
                },
                header: {
                    "Content-Type": "application/json"
                },
                success: (res) => {
                    if (res.data.code == 200 ) {
                        resolve();
                   
                    }
                },
                fail: (res) => {
    
                }
            })
        })
    },
    uploadError(e) {
        console.log('upload error', e.detail)
    },
    uploadSuccess(e) {
        console.log('upload success', e.detail)
    },

base64.js

var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",
  a256 = '',
  r64 = [256],
  r256 = [256],
  i = 0;

var UTF8 = {

  /**
   * Encode multi-byte Unicode string into utf-8 multiple single-byte characters
   * (BMP / basic multilingual plane only)
   *
   * Chars in range U+0080 - U+07FF are encoded in 2 chars, U+0800 - U+FFFF in 3 chars
   *
   * @param {String} strUni Unicode string to be encoded as UTF-8
   * @returns {String} encoded string
   */
  encode: function (strUni) {
    // use regular expressions & String.replace callback function for better efficiency
    // than procedural approaches
    var strUtf = strUni.replace(/[\u0080-\u07ff]/g, // U+0080 - U+07FF => 2 bytes 110yyyyy, 10zzzzzz
      function (c) {
        var cc = c.charCodeAt(0);
        return String.fromCharCode(0xc0 | cc >> 6, 0x80 | cc & 0x3f);
      })
      .replace(/[\u0800-\uffff]/g, // U+0800 - U+FFFF => 3 bytes 1110xxxx, 10yyyyyy, 10zzzzzz
        function (c) {
          var cc = c.charCodeAt(0);
          return String.fromCharCode(0xe0 | cc >> 12, 0x80 | cc >> 6 & 0x3F, 0x80 | cc & 0x3f);
        });
    return strUtf;
  },

  /**
   * Decode utf-8 encoded string back into multi-byte Unicode characters
   *
   * @param {String} strUtf UTF-8 string to be decoded back to Unicode
   * @returns {String} decoded string
   */
  decode: function (strUtf) {
    // note: decode 3-byte chars first as decoded 2-byte strings could appear to be 3-byte char!
    var strUni = strUtf.replace(/[\u00e0-\u00ef][\u0080-\u00bf][\u0080-\u00bf]/g, // 3-byte chars
      function (c) { // (note parentheses for precence)
        var cc = ((c.charCodeAt(0) & 0x0f) << 12) | ((c.charCodeAt(1) & 0x3f) << 6) | (c.charCodeAt(2) & 0x3f);
        return String.fromCharCode(cc);
      })
      .replace(/[\u00c0-\u00df][\u0080-\u00bf]/g, // 2-byte chars
        function (c) { // (note parentheses for precence)
          var cc = (c.charCodeAt(0) & 0x1f) << 6 | c.charCodeAt(1) & 0x3f;
          return String.fromCharCode(cc);
        });
    return strUni;
  }
};

while (i < 256) {
  var c = String.fromCharCode(i);
  a256 += c;
  r256[i] = i;
  r64[i] = b64.indexOf(c);
  ++i;
}

function code(s, discard, alpha, beta, w1, w2) {
  s = String(s);
  var buffer = 0,
    i = 0,
    length = s.length,
    result = '',
    bitsInBuffer = 0;

  while (i < length) {
    var c = s.charCodeAt(i);
    c = c < 256 ? alpha[c] : -1;

    buffer = (buffer << w1) + c;
    bitsInBuffer += w1;

    while (bitsInBuffer >= w2) {
      bitsInBuffer -= w2;
      var tmp = buffer >> bitsInBuffer;
      result += beta.charAt(tmp);
      buffer ^= tmp << bitsInBuffer;
    }
    ++i;
  }
  if (!discard && bitsInBuffer > 0) result += beta.charAt(buffer << (w2 - bitsInBuffer));
  return result;
}

var Plugin = function (dir, input, encode) {
  return input ? Plugin[dir](input, encode) : dir ? null : this;
};

Plugin.btoa = Plugin.encode = function (plain, utf8encode) {
  plain = Plugin.raw === false || Plugin.utf8encode || utf8encode ? UTF8.encode(plain) : plain;
  plain = code(plain, false, r256, b64, 8, 6);
  return plain + '===='.slice((plain.length % 4) || 4);
};

Plugin.atob = Plugin.decode = function (coded, utf8decode) {
  coded = coded.replace(/[^A-Za-z0-9\+\/\=]/g, "");
  coded = String(coded).split('=');
  var i = coded.length;
  do {
    --i;
    coded[i] = code(coded[i], true, r64, a256, 6, 8);
  } while (i > 0);
  coded = coded.join('');
  return Plugin.raw === false || Plugin.utf8decode || utf8decode ? UTF8.decode(coded) : coded;
};
module.exports = {
  btoa: Plugin.btoa,
  atob: Plugin.atob
}