agree with qqchen79(知秋一叶 [MSMVP]), you can check the information in the registry, but not every file type has a content type, HOW TO: work with the Windows Registry to determine the content-type of a file based on the file extension http://www.aspemporium.com/aspEmporium/cshrp/howtos/howto.asp?hid=19
HKEY_CLASSES_ROOT\.xls[Content Type=...]
HOW TO: work with the Windows Registry to determine the content-type of a file based on the file extension
http://www.aspemporium.com/aspEmporium/cshrp/howtos/howto.asp?hid=19