为什么ace：fileEntry不起作用？

Question

我正在使用jsf 2.0和icefaces 3组件上传文件：

<h:form enctype="multipart/form-data">   
<ace:fileEntry
        id="file-entry"
        relativePath="/files/"
        maxFileCount="10"
        maxFileCountMessage="Limited to 10 files uploaded concurrantly."
        fileEntryListener="#{uploadFileBean.sampleListener}"
        maxFileSize="6291456"
        maxFileSizeMessage="Submitted file is too large."
        maxTotalSize="18874368"
        maxTotalSizeMessage="Total size of submitted files is too large."
        required="true"
        requiredMessage="The file is required to submit this form."
        useOriginalFilename="true"
        useSessionSubdir="true" />

      <h:commandButton
        id="submit"
        type="submit"
        value="Send File" />
      </h:form>

但在我选择要上传的文件并按Send File按钮后，我会收到：

The file is required to submit this form.

这很奇怪，因为我已经选择了一个文件。

有没有人有任何建议？