const adTypeBinary=1

dim jpg(1):jpg(0)=CByte(HFF):jpg(1)=CByte(HD8)
dim bmp(1):bmp(0)=CByte(H42):bmp(1)=CByte(H4D)
dim png(3):png(0)=CByte(H89):png(1)=CByte(H50):png(2)=CByte(H4E):png(3)=CByte(H47)
dim gif(5):gif(0)=CByte(H47):gif(1)=CByte(H49):gif(2)=CByte(H46):gif(3)=CByte(H39):gif(4)=CByte(H38):gif(5)=CByte(H61)
Response.Write CheckFileType(Server.MapPath("2.gif"))

function CheckFileType(filename)
on error resume next
CheckFileType=false
dim fstream,fileExt,stamp,i
fileExt=mid(filename,InStrRev(filename,".")+1)
set fstream=Server.createobject("ADODB.Stream")
fstream.Open
fstream.Type=adTypeBinary
fstream.LoadFromFile filename
fstream.position=0
select case fileExt
case "jpg","jpeg"
stamp=fstream.read(2)
for i=0 to 1
if ascB(MidB(stamp,i+1,1))=jpg(i) then CheckFileType=true else CheckFileType=false
next
case "gif"
stamp=fstream.read(6)
for i=0 to 5
if ascB(MidB(stamp,i+1,1))=gif(i) then CheckFileType=true else CheckFileType=false
next
case "png"
stamp=fstream.read(4)
for i=0 to 3
if ascB(MidB(stamp,i+1,1))=png(i) then CheckFileType=true else CheckFileType=false
next
case "bmp"
stamp=fstream.read(2)
for i=0 to 1
if ascB(MidB(stamp,i+1,1))=bmp(i) then CheckFileType=true else CheckFileType=false
next
end select
fstream.Close
set fseteam=nothing
if err.number>0 then CheckFileType=false
end function
%>