ASP code for preventing File Upload Trojans

The ASP Trojan defense program code is as follows:
<%
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, limit Rev (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
%>