<el-upload
            :show-file-list="false"
            :on-success="handleAvatarSuccess"
            :before-upload="beforeAvatarUpload"
            :data="uploadToken"
            class="avatar-uploader"
            accept=".jpg,.jpeg,.png"
            action="https://upload.qiniup.com">
            <!-- http://up.qiniup.com -->
            <img
              v-if="form.avatar"
              :src="form.avatar"
              class="avatar">
            <img
              v-else
              src="~/assets/images/user-logo.png"
              class="avatar">
          </el-upload>

使用accept，同时要加以格式判断

    beforeAvatarUpload(file) {
      console.log('file', file)
      let types = ['image/jpeg', 'image/jpg', 'image/png'];
      const isImage = types.includes(file.type);
      if (!isImage) {
        this.$message.error('上传图片只能是 JPG、JPEG、PNG 格式!');
      }else{
        this.uploadToken.key=`upload_pic_${Date.parse(new Date())}${file.name}`
      }
    },

const isLtSize = file.size / 1024 / 1024 < 5;
 if (!isLtSize) {
          this.$message.error('上传图片大小不能超过 5MB!');
        }