在表单中检查缺少的文件

Question

摘要：我想知道何时提交空白HTML文件字段，但是我的代码似乎只能捕获附加了文件的文件。有什么解决方法吗？

我有一个表单，允许用户提交包含一些有关它们的元数据的文件，如下所示：

<form method="post" action="/upload" enctype="multipart/form-data">
 <label>
      Paper abstract: 
      <input type="text" name="abstract" />
 </label>

 <label>
     <div class="item">Upload paper in PDF format only</div>
     <input type="file" name="file" />
 </label>

 <!-- the interface allows the html above to be duplicated and submit more papers, e.g., -->

 <label>
      Paper abstract: 
      <input type="text" name="abstract" />
 </label>

 <label>
     <div class="item">Upload paper in PDF format only</div>
     <input type="file" name="file" />
 </label>

<button type="submit">Save</button>
</form>

服务器代码处理此类帖子：

func Upload(r *http.Request){

    papers := len(r.Form["abstract"])//Check how many papers the user is submitting

    r.ParseMultipartForm(config.MaxMemory)

    for i := 0; i < papers; i++{

      //Saves file and returns bytes -- implemented elsewhere
      //This line will fail unless every file field has an attachment.
      //I'd like to see blank file fields too.
      path, fileSize := uploadFile(r.MultipartForm.File["file"][i])

      paper := &Paper{//struct defined elsewhere
         Abstract: r.Form["abstract"][i]
         Path: path, 
         Size: size,
      }

      savePaper(paper) //implemented elsewhere

    }
}

感谢任何提示。