FileServer handles not found css with MIME type error instead of 404

Question

I’m running a basic http.FileServer to serve a static site and I’ve come across an issue where requests for css files that don’t exist are getting canceled with a MIME type error:

Refused to apply style from 'http://localhost:8080/assets/main.css' because its MIME type ('text/plain')

Ideally, I’d prefer it to be handled with a 404 error as thats what it should actually be. Any possible work arounds I could try?

Answer 1

From the net/http source code (fs.go):

// toHTTPError returns a non-specific HTTP error message and status code
// for a given non-nil error value. It's important that toHTTPError does not
// actually return err.Error(), since msg and httpStatus are returned to users,
// and historically Go's ServeContent always returned just "404 Not Found" for
// all errors. We don't want to start leaking information in error messages.
func toHTTPError(err error) (msg string, httpStatus int) {
    if os.IsNotExist(err) {
        return "404 page not found", StatusNotFound
    }
    if os.IsPermission(err) {
        return "403 Forbidden", StatusForbidden
    }
    // Default:
    return "500 Internal Server Error", StatusInternalServerError
}

The file server returns a 200 with a plain text file for 404 errors. The browser tries to interpret this plain text error page as a CSS file, and throws the error.

This behavior of returning the plain text file cannot be overridden on the handler returned by FileServer().

As has been pointed out already, this isn't really a bug in net/http.

If this behavior is undesirable for you for some reason, you can explore creating a custom handler for 404 responses, which has been explored in this thread. You could also use a routing library like Gorilla which has overridable behavior for not found pages.