el-upload自定义上传只能上传一次，无法上传多次

 upload
            class="upload-demo"
            action
            multiple
            :limit="1"
            ref="uploadPDF"
            :file-list="fileListPDF"
            :before-upload="beforeAvatarUploadPDF"
            :on-remove="handleRemovePDF"
            :show-file-list="false"
            :on-success="handleReplaceAvatarSuccessPDF"
            :http-request="handleHttpRequestPDF"
          >

在beforeAvatarUploadPDF方法执行完成后