Fix TestFile.test_markdown on Python 3.12.5

mime type mapping for markdown was added.

https://docs.python.org/3.12/whatsnew/changelog.html#id3

tests/checker/data/file-octet-stream.markdown

@@ -0,0 +1,18 @@
+# Test #
+
+<http://url.example.com>  text <http://url2.example.com>
+
+[link](     http://urllink.example.com)
+
+[link2](http://urllink2
+  .example.com)
+
+[test][id1]
+[URL][id2]
+
+[id1]:       
+http://urldef1.example.com
+
+[id2]: http://urldef2.example.com "URL"
+
+![img](http://urlimg.example.com)

tests/checker/data/file-octet-stream.markdown.result

@@ -0,0 +1,42 @@
+url file://%(curdir)s/%(datadir)s/file-octet-stream.markdown
+cache key file://%(curdir)s/%(datadir)s/file-octet-stream.markdown
+real url file://%(curdir)s/%(datadir)s/file-octet-stream.markdown
+name %(datadir)s/file-octet-stream.markdown
+warning The URL with content type 'application/octet-stream' is not parseable.
+valid
+
+url http://url.example.com
+cache key http://url.example.com
+real url http://url.example.com
+error
+
+url http://url2.example.com
+cache key http://url2.example.com
+real url http://url2.example.com
+error
+
+url http://urldef1.example.com
+cache key http://urldef1.example.com
+real url http://urldef1.example.com
+error
+
+url http://urldef2.example.com
+cache key http://urldef2.example.com
+real url http://urldef2.example.com
+error
+
+url http://urllink.example.com
+cache key http://urllink.example.com
+real url http://urllink.example.com
+error
+
+url http://urllink2.example.com
+cache key http://urllink2.example.com
+real url http://urllink2.example.com
+error
+
+url http://urlimg.example.com
+cache key http://urlimg.example.com
+real url http://urlimg.example.com
+error
+

tests/checker/data/file.markdown.result

@@ -2,7 +2,7 @@ url file://%(curdir)s/%(datadir)s/file.markdown
 cache key file://%(curdir)s/%(datadir)s/file.markdown
 real url file://%(curdir)s/%(datadir)s/file.markdown
 name %(datadir)s/file.markdown
-warning The URL with content type 'application/octet-stream' is not parseable.
+warning The URL with content type 'text/markdown' is not parseable.
 valid
 
 url http://url.example.com

tests/checker/test_file.py

@@ -87,7 +87,11 @@ class TestFile(LinkCheckTest):
 
     def test_markdown(self):
         confargs = dict(enabledplugins=["MarkdownCheck"])
-        self.file_test("file.markdown", confargs=confargs)
+        if sys.version_info < (3, 12, 5):
+            markdown_file = "file-octet-stream.markdown"
+        else:
+            markdown_file = "file.markdown"
+        self.file_test(markdown_file, confargs=confargs)
 
     def test_urllist(self):
         self.file_test("urllist.txt")