This function checks the uploaded file contents to ensure that the file is an image. You can check extension of the file or content-type. If the content-type begins with 'image/' or extension is in imageExts, the file is probably an image.
     But - someone can rename an .exe file and send the file as 'image'. So a second part of the check function tests file contents for usual characteristics of image files. You can use String or HexString property to get a part of contents of the file and compare it with image file characteristics.
     You can use this function to check other file types (compressed, executable, …) also.
These tables are from Richard Ogley's pages.

Jump to: Image files | Compressed files | Archive files | Executable files | Miscellaneous files

Image files

File type	Typical extension	Hex digits xx = variable	Ascii digits . = not an ascii char
Bitmap format	.bmp	42 4d	BM
FITS format	.fits	53 49 4d 50 4c 45	SIMPLE
GIF format	.gif	47 49 46 38	GIF8
Graphics Kernel System	.gks	47 4b 53 4d	GKSM
IRIS rgb format	.rgb	01 da	..
ITC (CMU WM) format	.itc	f1 00 40 bb	....
JPEG File Interchange Format	.jpg	ff d8 ff e0	....
NIFF (Navy TIFF)	.nif	49 49 4e 31	IIN1
PM format	.pm	56 49 45 57	VIEW
PNG format	.png	89 50 4e 47	.PNG
Postscript format	.[e]ps	25 21	%!
Sun Rasterfile	.ras	59 a6 6a 95	Y.j.
Targa format	.tga	xx xx xx	...
TIFF format (Motorola - big endian)	.tif	4d 4d 00 2a	MM.*
TIFF format (Intel - little endian)	.tif	49 49 2a 00	II*.
X11 Bitmap format	.xbm	xx xx
XCF Gimp file structure	.xcf	67 69 6d 70 20 78 63 66 20 76	gimp xcf
Xfig format	.fig	23 46 49 47	#FIG
XPM format	.xpm	2f 2a 20 58 50 4d 20 2a 2f	/* XPM */

Compressed files

File type	Typical extension	Hex digits xx = variable	Ascii digits . = not an ascii char
Bzip	.bz	42 5a	BZ
Compress	.Z	1f 9d	..
gzip format	.gz	1f 8b	..
pkzip format	.zip	50 4b 03 04	PK..

Archive files

File type	Typical extension	Hex digits xx = variable	Ascii digits . = not an ascii char
TAR (pre-POSIX)	.tar	xx xx	(a filename)
TAR (POSIX)	.tar	75 73 74 61 72	ustar (offset by 257 bytes)

Excecutable files

File type	Typical extension	Hex digits xx = variable	Ascii digits . = not an ascii char
MS-DOS, OS/2 or MS Windows		4d 5a	MZ
Unix elf		7f 45 4c 46	.ELF

Miscellaneous files

File type	Typical extension	Hex digits xx = variable	Ascii digits . = not an ascii char
pgp public ring		99 00	..
pgp security ring		95 01	..
pgp security ring		95 00	..
pgp encrypted data		a6 00	¦.

Upload - Check file contents to ensure the field is really an image
Const imageExts = ".gif,.jpg,.png,.jpeg,.bmp" … 'This function checks filename and CONTENTS of a field 'to recognize images Function IsImage(Field) If Instr(1, imageExts & ",", Field.FileExt & ",", 1)>0 _ Or Left(Field.ContentType, 5) = "image" Then ' FFD8FF = JFIF ' 49492A00 = TIF If Field.HexString (0,3)="FFD8FF" Or Field.HexString (0,4)="49492A00" _ Or Field.String(,6,4)="JFIF" Or Field.String(,0,3)="GIF" _ Or Field.String(,1,3)="PNG" Or Field.String(,0,2)="BM" Then IsImage = True End If End If End Functio