Set Default Content Type in http.server in Python 3.15

The http.server module decides the Content-Type header of a response from the file extension, using the mimetypes database. When the extension is not recognized, it falls back to application/octet-stream, which browsers treat as a download rather than something to display. Files with custom extensions, such as logs, data dumps or configuration written in a project-specific format, are therefore saved to disk instead of being shown.

Create an empty file notes.bin2 in the current directory and start the server:

python -m http.server 8000

curl -I http://127.0.0.1:8000/notes.bin2

HTTP/1.0 200 OK
Server: SimpleHTTP/0.6 Python/3.14.10
Date: Tue, 03 Nov 2026 09:41:47 GMT
Content-type: application/octet-stream
Content-Length: 0
Last-Modified: Tue, 03 Nov 2026 09:24:03 GMT

Since Python 3.15, the --content-type option sets the type used for extensions which are not recognized:

python -m http.server 8000 --content-type text/plain

HTTP/1.0 200 OK
Server: SimpleHTTP Python/3.15.2
Date: Tue, 03 Nov 2026 09:41:47 GMT
Content-type: text/plain
Content-Length: 0
Last-Modified: Tue, 03 Nov 2026 09:24:03 GMT

The option applies only to unrecognized extensions. A request for index.html still returns text/html, because the extension is resolved and the fallback is never reached.

The same value can be set from Python with default_content_type attribute of SimpleHTTPRequestHandler:

import http.server
import socketserver


class Handler(http.server.SimpleHTTPRequestHandler):
    default_content_type = 'text/plain'


with socketserver.TCPServer(('', 8000), Handler) as httpd:
    httpd.serve_forever()

The attribute is defined on the class with a default of application/octet-stream, so a subclass can replace it directly. It is not a constructor argument, and passing it as one raises TypeError.