axios 将post请求数据转为formdata

axios({
            url: '/api/index/getIndexlbt',
            method: 'post',
            data: {
              relevanceId:this.$route.params.id,
              pictureType:4},
            transformRequest: [function(data) {
              let ret = ''
              for (let it indata) {
                ret += encodeURIComponent(it) + '=' + encodeURIComponent(data[it]) + '&'}
              returnret
            }],
            headers: {
              'Content-Type': 'application/x-www-form-urlencoded'}
          })