ICO MIME type: image/vnd.microsoft.icon

The registered MIME type for .ico files is image/vnd.microsoft.icon. image/x-icon is the older unofficial name and still the more common one: it’s what nginx and Apache ship in their mime.types, and what the MIME Sniffing standard calls the format. The tool gives both, registered name first:

image/vnd.microsoft.icon   Icon or cursor image (registered name)
.ico
Content-Type: image/vnd.microsoft.icon

image/x-icon   Icon or cursor image (unofficial name)   Unofficial
.ico .cur
Content-Type: image/x-icon

Which one to send

Either works. Browsers identify icons from their bytes: an ICO file starts with 00 00 01 00, and a .cur cursor with 00 00 02 00. Both signatures are in the MIME Sniffing standard’s image table, labeled image/x-icon. There’s nothing to fix if your server sends one or the other; for a new configuration, the registered name is the tidier choice.

An ICO file can hold several sizes, such as 16, 32 and 48 pixels, and its images can be stored as PNG inside; the type stays the same. A modern favicon set often pairs it with an SVG and a PNG for Apple devices:

<link rel="icon" href="/favicon.ico" sizes="32x32">
<link rel="icon" href="/icon.svg" type="image/svg+xml">
<link rel="apple-touch-icon" href="/apple-touch-icon.png">

The type attribute is a hint that lets a browser skip an icon format it can’t use.

Gotchas

• Browsers ask for /favicon.ico on their own when a page declares no icon, so a missing file shows up as a steady stream of 404s in the logs.
• Python’s answer depends on the machine. Its built-in table maps .ico to image/vnd.microsoft.icon, but mimetypes also reads system files such as /etc/mime.types, which override it. On macOS, Apache’s file there turns the answer into image/x-icon. aws s3 cp guesses types this way, so the same upload can get either name.

In your stack

nginx and Apache’s bundled mime.types send image/x-icon, Express 4 sends image/x-icon, and Express 5 sends image/vnd.microsoft.icon. To use the registered name:

# http block, after include mime.types; nginx warns about the duplicate ico
types {
    image/vnd.microsoft.icon  ico;
}

AddType image/vnd.microsoft.icon .ico

aws s3 cp favicon.ico s3://my-bucket/favicon.ico \
  --content-type image/vnd.microsoft.icon --cache-control 'max-age=86400'