vue上传单张图片

效果如下，图片可点击替换：

<el-form-item label=" " style="margin-left:4px;height:70px">
      <div class="el-input el-input-group el-input-group--prepend addprepend" style="height:100%">
        <div class="el-input-group__prepend addPrependText-medium" tabindex="-1" style="height:100%">
          <span>海报图片</span>
        </div>
        <el-upload
          :action="uploadImg"
          :on-preview="handlePreview"
          :before-upload="beforeAvatarUpload"
          :on-success="handleAvatarSuccess"
          :on-remove="handleRemove">
          <div style="border: 1px solid #ccc;padding: 10px;height:100%;">
            <div v-if="!form.img" style="200px"><i class="el-icon-plus" style="margin-top:4%"/></div>
            <img v-else :src="form.img" style="200px">
          </div>
        </el-upload>
      </div>
      <span style="color:red;font-size:12px">推荐尺寸750*750</span>
    </el-form-item>

data() {
    return {
      form: {
        name: '',
        category: '',
        img: ''
      },
      fileList: [],
      uploadImg: process.env.BASE_API + '/management/fileupload/qiniu/img'
    }

iniData() {          
      if (!this.selectId) {        //添加初始化
        this.$axios
          .get('base/posters/create')
          .then(res => {
            this.form = res.content.poster
            this.types = res.content.categoryList
          })
          .catch(res => {})
      } else {                   //编辑初始化
        this.$axios
          .get('base/posters/' + this.selectId + '/update')
          .then(res => {
            this.types = res.content.categoryList
            res.content.poster.category = res.content.poster.categoryId
            this.form = res.content.poster
            this.fileList = [{ name: '', url: this.form.img }]
          })
          .catch(res => {})
      }

beforeAvatarUpload(file) {
      console.log(file)
      const isJPG = file.type
      console.log(file.type)
      const isLt2M = file.size / 1024 / 1024 < 2
      if (
        isJPG != 'image/jpg' &&
        isJPG != 'image/png' &&
        isJPG != 'image/jpeg' &&
        isJPG != 'image/gif'
      ) {
        this.$message.error('只能上传.jpg  .png  .jpeg  .gif类型的文件!')
      }
      if (!isLt2M) {
        this.$message.error('上传图片大小不能超过 2MB!')
      }
      return isJPG && isLt2M
    },
    handlePreview(file) {
      console.log(file)
    },
    handleRemove(file) {
      console.log(file)
    },
    handleAvatarSuccess(res, file) {
      console.log('=======================', res)
      this.form.img = res.content.accessUrl
      this.img = [{ name: '海报图片.png', url: res.content.accessUrl }]
    },