Hi . this code may help you . i don't test them ! ;)
for save file use this :
FileUpload1.SaveAs(Server.MapPath("Books/") +FileUpload1.FileName);
for file-type check use this that return the answer ( seem better that switch!) :
if(
(System.IO.Path.GetExtension(FileUpload1.FileName) == ".jpg") || (System.IO.Path.GetExtension(FileUpload1.FileName) == ".jpeg") || (System.IO.Path.GetExtension(FileUpload1.FileName) == ".bmp") || (System.IO.Path.GetExtension(FileUpload1.FileName) == ".gif") || (System.IO.Path.GetExtension(FileUpload1.FileName) == ".ico") || (System.IO.Path.GetExtension(FileUpload1.FileName) == ".png")
)
{some code}
else
{some code}