File Signatures (Magic Numbers)

Relying solely on file extensions to determine file formats is unreliable. Instead, systems and applications often inspect the raw bytes at specific offsets within a file header to identify its true format. These unique byte sequences are known as magic numbers or file signatures. A comprehensive resource for these signatures is maintained by Gary Kessler.

Below is a restructured mapping of common file signatures. Instead of formatting offsets and hex sequences as comma-separated strings, the data is organized into explicit objects for better parsing and readability:

{
  "image/jpeg": {
    "extensions": ["jpg", "jpeg"],
    "signatures": [
      { "offset": 0, "hex": "FFD8" }
    ]
  },
  "image/png": {
    "extensions": ["png"],
    "signatures": [
      { "offset": 0, "hex": "89504E470D0A1A0A" }
    ]
  },
  "image/gif": {
    "extensions": ["gif"],
    "signatures": [
      { "offset": 0, "hex": "474946383961" }
    ]
  },
  "application/pdf": {
    "extensions": ["pdf"],
    "signatures": [
      { "offset": 0, "hex": "25504446" }
    ]
  },
  "application/zip": {
    "extensions": ["zip", "docx", "xlsx", "pptx", "odt"],
    "signatures": [
      { "offset": 0, "hex": "504B0304" }
    ]
  },
  "application/x-msdownload": {
    "extensions": ["exe", "dll"],
    "signatures": [
      { "offset": 0, "hex": "4D5A" }
    ]
  },
  "application/gzip": {
    "extensions": ["gz", "tgz"],
    "signatures": [
      { "offset": 0, "hex": "1F8B08" }
    ]
  },
  "application/x-rar-compressed": {
    "extensions": ["rar"],
    "signatures": [
      { "offset": 0, "hex": "526172211A0700" }
    ]
  },
  "audio/mpeg": {
    "extensions": ["mp3"],
    "signatures": [
      { "offset": 0, "hex": "FFFB" },
      { "offset": 0, "hex": "494433" }
    ]
  },
  "video/webm": {
    "extensions": ["webm"],
    "signatures": [
      { "offset": 0, "hex": "1A45DFA3" }
    ]
  }
}

MIME Types Configuration

When web servers like Nginx or Apache serve a file, they include a Content-Type HTTP header to inform the browser about the nature of the data. This mapping between file extensions and MIME types is defined in configuration files such as mime.types.

The official registry for Internet Media Types is maintained by IANA, following the registration guidelines established in RFC 4288 and the syntax rules defined in RFC 6838.

types {
    text/html                                        html htm shtml;
    text/css                                         css;
    text/xml                                         xml;
    application/json                                 json;
    application/javascript                           js;
    application/atom+xml                             atom;
    application/rss+xml                              rss;

    image/jpeg                                       jpeg jpg;
    image/png                                        png;
    image/gif                                        gif;
    image/svg+xml                                    svg svgz;
    image/webp                                       webp;
    image/x-icon                                     ico;

    application/octet-stream                         bin exe dll;
    application/pdf                                  pdf;
    application/zip                                  zip;
    application/x-rar-compressed                     rar;
    application/x-7z-compressed                      7z;
    application/gzip                                 gz;

    application/vnd.ms-excel                         xls;
    application/vnd.openxmlformats-officedocument.spreadsheetml.sheet xlsx;
    application/vnd.ms-powerpoint                    ppt;
    application/vnd.openxmlformats-officedocument.presentationml.presentation pptx;
    application/msword                               doc;
    application/vnd.openxmlformats-officedocument.wordprocessingml.document docx;

    audio/mpeg                                       mp3;
    audio/ogg                                        ogg;
    video/mp4                                        mp4;
    video/webm                                       webm;
    video/quicktime                                  mov;
}