Upload - Check file contents to ensure the field is really an image

Examples

     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

Compressed files

Archive files

Excecutable files

Miscellaneous files

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

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