预览已经上传的图片要点：
window.URL.createObjectURL(new Blob([res]))

上传图片的要点：
 const formData = new FormData()
 formData.append('file', file.file)
 
完整案例：
1、预览图片：
   <a-form-item
    class="ef-item ef-i-w100"
    label="姓名"
    :labelCol="labelCol"
    :wrapperCol="wrapperCol"
    has-feedback
  >
    <template>
      <a-auto-complete placeholder="请输入姓名" allowClear @search="handleSearch" @change="handleSelect($event,`${item.id}`)">
        <template slot="dataSource">
          <a-select-option v-for="(it, index) in result" :key="index" :value="it.id">
            {{ it.policeName }}
          </a-select-option>
        </template>
      </a-auto-complete>
    </template>
 
    <a-input 
    v-show="false"         
    v-decorator="['policeName', {rules: [{
        required: true,
        validator: (rule, value, callBack)=>{
          var Reg=/^[\u2E80-\u9FFF]+$/;//Unicode编码中的汉字范围
          if(!value){
            callBack('请输入姓名')
          }else if(!Reg.test(value)) {
            callBack('输入姓名不合法')
          }
          callBack()
        }
      }]
      }
      ]"
    ></a-input>
 </a-form-item>
 
 <a-form-item
      class="ef-item ef-i-w50 phones"
      label="照片"
      :labelCol="labelCol"
      :wrapperCol="wrapperCol"
      has-feedback
    >
      <div class="ant-upload-preview" @click="edit(item.id)" v-decorator="['poliecePhoto']">
        <div class="mask">
          <a-icon type="plus" />
        </div>
        <img :src="option[item.id].img == null ? option[0].img :option[item.id].img"/>
      </div>
</a-form-item>

//根据id预览图片
 handleSelect(value,index) {
        if(this.result == null && this.result.length === 0){return}
        const filterValue = this.result.filter(item => item.id === value)
        const { form } = this.formList[index]
        if(filterValue!= null && filterValue.length > 0) {
          const record = filterValue[0]
          form.setFieldsValue({
              policeName: record.policeName,
              ...
            })

            //预览照片
          this.businessEscortPreview(record, index)

        } else {
          form.setFieldsValue({
              policeName: '',
              ....
            })
        }

      },
     /**
       * 获取图片并转为链接
      */
      businessEscortPreview(record, index) {
        businessEscortPreview({ id: record.poliecePhoto })
          .then((res) => {
            const src = window.URL.createObjectURL(new Blob([res]))
            this.option[index].img = res.byteLength <= 208 ? '/default_mj_photo.png' : src          
          })
          .catch((err) => {
            this.$message.error('预览错误：' + err.message)
          })
      },
      
      
 2、扩展知识：上传图片 
//上传图片 大致思路 
  <a-upload
      name="file"
      ref="files"
      :multiple="false"
      :file-list="fileList"
      :beforeUpload="beforeUpload"
      :remove="handleRemove"
      @change="handleChangeFile"
      @preview="handlePreview"
    >
      <a-button class="buttons" :disabled="disableType"> 上传附件 </a-button>  
</a-upload>

//上传当前图片,每次校验完毕及时上传，需要使用formData 方式
handleChangeFile(file) {
  if (this.fileStatus != 'removed') {
    const { $message } = this
    const formData = new FormData()
    formData.append('file', file.file)
    this.fileList = file.fileList

    sysFileInfoUpload(formData)
      .then((res) => {
        if (res.code == 200) {
          $message.success('上传成功')
          //res.data 即为上传图片生成的id
          this.adjunctArr.push(res.data)
          this.adjunct = this.adjunctArr.join(',')
          this.$emit('adjunctFun', {
            adjunct: this.adjunct,
          })
        }
      })
      .catch((err) => {
        $message.error('上传失败' + err.message)
      })
  }
},