without global mimetype override

django/utils/feedgenerator.py

@@ -58,26 +58,32 @@ def get_tag_uri(url, date):
     return "tag:%s%s:%s/%s" % (bits.hostname, d, bits.path, bits.fragment)
 
 
+def _guess_stylesheet_mimetype(url):
+    """
+    Return the given stylesheet's mimetype tuple, using a slightly custom
+    version of Python's mimetypes.guess_type().
+    """
+    mimetypedb = mimetypes.MimeTypes()
+
+    # The official mimetype for XSLT files is technically `application/xslt+xml`
+    # but as of 2024 almost no browser supports that (they all expect text/xsl).
+    # On top of that, windows seems to assume that the type for xsl is text/xml.
+    mimetypedb.readfp(StringIO("text/xsl\txsl\ntext/xsl\txslt"))
+
+    return mimetypedb.guess_type(url)
+
+
 NOT_PROVIDED = object()
 
 
 class Stylesheet:
     """An RSS stylesheet"""
 
-    MIMETYPE_OVERRIDES = {
-        # The official mimetype for XSLT files is technically `application/xslt+xml`
-        # but as of 2024 almost no browser support that.
-        "application/xslt+xml": "text/xsl",
-    }
-
     def __init__(self, url, mimetype=NOT_PROVIDED, media="screen"):
         self.url = iri_to_uri(url)
 
         if mimetype is NOT_PROVIDED:
-            mimetypes.add_type("application/xslt+xml", ".xsl")
-            mimetypes.add_type("application/xslt+xml", ".xslt")
-            mimetype, _ = mimetypes.guess_type(self.url)
-            mimetype = self.MIMETYPE_OVERRIDES.get(mimetype, mimetype)
+            mimetype, _ = _guess_stylesheet_mimetype(self.url)
 
         self.mimetype = mimetype
         self.media = media