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Home Interviews How do you check if a file is an image, PDF, or audio file by inspecting its content? – MindStick
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How do you check if a file is an image, PDF, or audio file by inspecting its content?

ICSM Computer 530 16 May 2025
c# c#
ICSM Computer
Ask by ICSM Computer
Updated 16 May 2025

1 Answers

ICSM Computer
ICSM Computer 16 May 2025
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To check if a file is an image, PDF, or audio file by inspecting its content, you should analyze the file signature (also known as the "magic number")—a specific byte pattern at the beginning of the file that indicates its type.

1. Read the File Header (Magic Number)

Here's a C# example:

public static string GetFileType(string filePath)
{
    byte[] header = new byte[8];
    using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
    {
        fs.Read(header, 0, header.Length);
    }

    // Check against known signatures
    if (header.Take(3).SequenceEqual(new byte[] { 0xFF, 0xD8, 0xFF }))
        return "JPEG image";

    if (header.Take(8).SequenceEqual(new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }))
        return "PNG image";

    if (header.Take(4).SequenceEqual(new byte[] { 0x25, 0x50, 0x44, 0x46 }))
        return "PDF document";

    if (header.Take(4).SequenceEqual(new byte[] { 0x49, 0x44, 0x33 })) // or use offset for ID3v2
        return "MP3 audio";

    if (header.Take(4).SequenceEqual(new byte[] { 0x52, 0x49, 0x46, 0x46 }))
        return "WAV or AVI (need more info)";

    return "Unknown";
}

2. Common Signatures

File Type Magic Number (Hex) Description
JPEG FF D8 FF First 3 bytes
PNG 89 50 4E 47 0D 0A 1A 0A First 8 bytes
PDF 25 50 44 46 %PDF
MP3 49 44 33 ID3 at start (ID3v2)
WAV 52 49 46 46 RIFF (check next bytes)
GIF 47 49 46 38 GIF8

To distinguish between WAV and AVI, you'd need to check the next few bytes after RIFF.

3. Notes

  1. This method is more secure and reliable than using file extensions.
  2. To support more formats, extend the signature table.
  3. This approach works well for validation before saving or processing files.
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