How can you detect file type using byte-level inspection (e.g., IsPdf, IsImage)?

Question

Anubhav Kumar · Answer

Detecting file types using byte-level inspection (also called
magic number checking) involves reading the beginning bytes of a file (the "magic number") to determine its format. This is more reliable than checking file extensions.
Here's how you can implement file type detection (e.g., IsPdf, 
IsImage, etc.) by inspecting the byte content:
Step-by-Step Overview
Read the header bytes of the file (usually the first 4–8 bytes).Match the byte signature against known file type "magic numbers."Optionally, allow some fuzziness for formats like image types with common variations.
Common File Type Magic Numbers
File Type
Magic Number (Hex)
ASCII
Description
PDF
25 50 44 46
%PDF
PDF files start with %PDF
PNG
89 50 4E 47 0D 0A 1A 0A
—
PNG image
JPEG
FF D8 FF
—
JPEG image
GIF
47 49 46 38
GIF8
GIF image (GIF87a/GIF89a)
ZIP
50 4B 03 04
PK..
ZIP archives (also DOCX, XLSX, etc.)
Example: Detect PDF and Image Types in C#
public static class FileTypeDetector
{
    public static bool IsPdf(byte[] bytes)
    {
        // Check if the first 4 bytes are "%PDF"
        return bytes.Length andgt;= 4 andand
               bytes[0] == 0x25 andand // %
               bytes[1] == 0x50 andand // P
               bytes[2] == 0x44 andand // D
               bytes[3] == 0x46;   // F
    }
    public static bool IsPng(byte[] bytes)
    {
        byte[] pngSignature = new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A };
        return bytes.Take(pngSignature.Length).SequenceEqual(pngSignature);
    }
    public static bool IsJpeg(byte[] bytes)
    {
        return bytes.Length andgt;= 3 andand
               bytes[0] == 0xFF andand
               bytes[1] == 0xD8 andand
               bytes[2] == 0xFF;
    }
    public static bool IsGif(byte[] bytes)
    {
        return bytes.Length andgt;= 4 andand
               bytes[0] == 0x47 andand // G
               bytes[1] == 0x49 andand // I
               bytes[2] == 0x46 andand // F
               bytes[3] == 0x38;   // 8
    }
    public static bool IsZip(byte[] bytes)
    {
        return bytes.Length andgt;= 4 andand
               bytes[0] == 0x50 andand // P
               bytes[1] == 0x4B andand // K
               bytes[2] == 0x03 andand
               bytes[3] == 0x04;
    }
    public static bool IsImage(byte[] bytes)
    {
        return IsPng(bytes) || IsJpeg(bytes) || IsGif(bytes);
    }
}
Usage Example
byte[] fileBytes = File.ReadAllBytes("example.pdf");
if (FileTypeDetector.IsPdf(fileBytes))
{
    Console.WriteLine("This is a PDF file.");
}
else if (FileTypeDetector.IsImage(fileBytes))
{
    Console.WriteLine("This is an image file.");
}
Benefits
Works regardless of file extension.Safer when dealing with user-uploaded files.Can easily be extended to support more file types.

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How can you detect file type using byte-level inspection (e.g., IsPdf, IsImage)?

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Step-by-Step Overview

Common File Type Magic Numbers

Example: Detect PDF and Image Types in C#

Usage Example

Benefits

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File Type	Magic Number (Hex)	ASCII	Description
PDF	`25 50 44 46`	`%PDF`	PDF files start with `%PDF`
PNG	`89 50 4E 47 0D 0A 1A 0A`	—	PNG image
JPEG	`FF D8 FF`	—	JPEG image
GIF	`47 49 46 38`	`GIF8`	GIF image (GIF87a/GIF89a)
ZIP	`50 4B 03 04`	`PK..`	ZIP archives (also DOCX, XLSX, etc.)