fileupload control in asp.net

File upload control doesn't have restriction for any file type. You can check the file type after uploaded or you can check the file extension using a javascript. But the later may not work in some browsers. Simple reason how a control can identify file type, though it may tell you the extension, extension doesn't make sure the file is that type. How about changing .xls to .doc and upload. So once the file is read i.e posted, you can get the file type for known types.

As long as the fileupload is a input file type in html, it has no logic to read a file before posting. So better verify the file type after posted.