Lesson 27 · Content-Type from file extension

Content-Type from file extension

A browser renders a file according to its Content-Type, so serving files means guessing the type from the extension. Today you map extensions to MIME types.

The goal

Choose a Content-Type header for a served file based on its filename extension.

Start here - the target

TO DO

Scenario: Mapping extensions to MIME types

Giventhe served file is "index.html"

Whenits content type is determined

Thenthe Content-Type is "text/html"

And".css" gives "text/css", ".txt" gives "text/plain", and an unknown ".xyz" gives "application/octet-stream"

Background

The bytes of a file do not say what they are — a browser decides whether to render <h1>Home</h1> as a heading or show it as source based entirely on the Content-Type you send. The conventional way to guess it is the filename extension: .html is text/html, .css is text/css, and so on.

Keep a small extension-to-MIME table and default anything unrecognized to application/octet-stream, the catch-all meaning “arbitrary bytes, let the client decide.” Set this header on the file responses from the previous lesson so pages and stylesheets render instead of downloading.

Make it work

var mimeTypes = map[string]string{
    ".html": "text/html", ".css": "text/css",
    ".txt": "text/plain", ".json": "application/json",
}
ct := mimeTypes[strings.ToLower(filepath.Ext(name))]
if ct == "" { ct = "application/octet-stream" }

CheckpointDONE

Served files carry a Content-Type guessed from their extension. Commit.