好的，我发现了问题。这不是一个Wordpress 问题，但服务器问题。

我必须添加文件扩展名.mp4 具有MIME类型video/mp4 在IIS服务器的MIME类型中。

以下是我找到的更正问题的步骤链接：http://forums.asp.net/t/1470612.aspx/1

 1) Select the site to configure in IIS, right click and select "Properties" 
 2) Under HTTP Headers Tab, select "File Types" under the MIME Map section and select "New Type" 
 3) Type ".flv" as the associated extension and "video/x-flv" as the content type or "flv-application/octet-stream" I\'m not sure on which one gives here.
 4) for .mp4 files type ".mp4" as the extension and "video/mp4" as the mime type (this one I tested personally) 
 5) Select "OK",  
 6) type services.msc, find the "World Wide Web Publishing Service" and click
 on the restart icon on top or open up and choose restart