addresses some of @encukou's observations. still need to add tests.

Author: jcomeauictx

Doc/library/http.server.rst

@@ -167,6 +167,13 @@ provides three different variants:
       header (using :meth:`send_header`) in all of its responses to clients.
       For backwards compatibility, the setting defaults to ``'HTTP/1.0'``.
 
+   .. attribute:: default_content_type
+
+      Specifies the content-type header value sent when the mime-type cannot
+      be guessed from the file extension of the requested URL.
+      For backwards compatibility, the setting defaults to
+      ``'application/octet-stream'``.
+
    .. attribute:: MessageClass
 
       Specifies an :class:`email.message.Message`\ -like class to parse HTTP
@@ -462,6 +469,16 @@ following command runs an HTTP/1.1 conformant server::
 .. versionchanged:: 3.11
    Added the ``--protocol`` option.
 
+By default, the server uses the MIME type ``'application/octet-stream'`` for
+the ``content-type`` header when the content type cannot be guessed from the
+URL's extension (if any). The option ``--default-content-type`` overrides
+this default with one of the user's choosing::
+
+        python -m http.server --default-content-type text/html
+
+.. versionchanged:: 3.13
+   Added the ``--default-content-type`` option.
+
 .. class:: CGIHTTPRequestHandler(request, client_address, server)
 
    This class is used to serve either files or output of CGI scripts from the

Lib/http/server.py

@@ -891,32 +891,17 @@ def guess_type(self, path):
         as a default; however it would be permissible (if
         slow) to look inside the data to make a better guess.
 
-        >>> testserver = type('', (), {
-        ...  'makefile': lambda mode, bufsize: open(os.devnull, mode)
-        ... })
-        >>> testhandler = SimpleHTTPRequestHandler(testserver, None, None)
-        >>> testhandler.guess_type('/foo.bar.GZ')  # tests ext.lower()
-        'application/gzip'
-        >>> testhandler.default_content_type = 'nonesuch/nonesuch'
-        >>> testhandler.guess_type('/this/should/give/default')
-        'nonesuch/nonesuch'
-        >>> # check short-circuiting works using mock mimetypes.guess_type
-        >>> sys.modules[__name__].mimetypes = type('', (), {
-        ...  'guess_type': lambda x: (print('foo'), print('bar'))
-        ... })
-        >>> testhandler.guess_type('/should/show/foo/bar/then/default')
-        foo
-        bar
-        'nonesuch/nonesuch'
-        >>> testhandler.guess_type('/should/not/print/foo.Z')
-        'application/octet-stream'
-
         """
         base, ext = posixpath.splitext(path)
-        return self.extensions_map.get(ext) or \
-            self.extensions_map.get(ext.lower()) or \
-            mimetypes.guess_type(path)[0] or \
-            self.default_content_type
+        if ext in self.extensions_map:
+            return self.extensions_map[ext]
+        ext = ext.lower()
+        if ext in self.extensions_map:
+            return self.extensions_map[ext]
+        guess, _ = mimetypes.guess_type(path)
+        if guess:
+            return guess
+        return self.default_content_type
 
 
 # Utilities for CGIHTTPRequestHandler