el-upload自定義上傳只能上傳一次，無法上傳多次

 <el-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方法執行完成后

執行this.$refs.uploadPDF.clearFiles();//在每一次上傳請求結束后清除緩存

即可上傳多次文件