文件下载时出现错误：
System.IO.DirectoryNotFoundException: Could not find a part of the path 'c:\webhosting\data\picture\upload\138\2015\07\21\138_1022240.jpg'.
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
at KeLin.WebSite.PageWap.ResponseFile(String realPath, String fileExt)