微信小程序获取文件session会话无效

微信小程序获取图片文件时session会话失效，可以将图片文件地址请求修改为post请求，返回文件流以base64编码输出。
具体请求方式如下：

const getImageCaptcha = ()=>{
    //return getApp().globalData.BASE_API+'/captcha?'+Math.random()
    let url = getApp().globalData.BASE_API+"/captcha?"+Math.random()
    return new Promise((resolve, reject) => {
        wx.request({
            url: url,
            method:'POST',
            header: {cookie:wx.getStorageSync('session_id')},
            responseType: 'arraybuffer',
            success:res=>{
                if(res.header['Set-Cookie']){
                    wx.setStorageSync('session_id', res.header['Set-Cookie']);
                } 
                let url = 'data:image/png;base64,' + wx.arrayBufferToBase64(res.data)
                resolve(url);
            }
        })
    })
}