判断文件真实的类型，不是通过扩展名来判断：

/// <summary>
/// 判断文件格式
/// http://www.cnblogs.com/babycool
/// </summary>
/// <param name="filePath"></param>
/// <returns></returns>
public static bool IsAllowedExtension(string filePath)
{

FileStream stream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
BinaryReader reader = new BinaryReader(stream);
string fileclass = "";
// byte buffer;
try
{

//buffer = reader.ReadByte();
//fileclass = buffer.ToString();
//buffer = reader.ReadByte();
//fileclass += buffer.ToString();

for (int i = 0; i < 2; i++)
{
fileclass += reader.ReadByte().ToString();
}

}
catch (Exception)
{

throw;
}

if (fileclass == "255216")
{
return true;
}
else
{
return false;
}

/*文件扩展名说明
* 255216 jpg
* 208207 doc xls ppt wps
* 8075 docx pptx xlsx zip
* 5150 txt
* 8297 rar
* 7790 exe
* 3780 pdf
*
* 4946/104116 txt
* 7173 gif
* 255216 jpg
* 13780 png
* 6677 bmp
* 239187 txt,aspx,asp,sql
* 208207 xls.doc.ppt
* 6063 xml
* 6033 htm,html
* 4742 js
* 8075 xlsx,zip,pptx,mmap,zip
* 8297 rar
* 01 accdb,mdb
* 7790 exe,dll
* 5666 psd
* 255254 rdp
* 10056 bt种子
* 64101 bat
* 4059 sgf
*/

}