fix(plain_text): partition is not distinguishing file extensions correctly

[Iced0368]

if replace_wikilinks:
            for w in parsed.wikilinks:
                b, e = w.span
                title = w.title
                if title[:1] != ':' and (
                    title.partition(':')[2].partition('.')[2]
                    in KNOWN_FILE_EXTENSIONS
                ):
                    remove(b, e)  # image

At line 743 of _wikitext.py, it calls partition to separate the extension from the file name. However, in this case, if the file name contains '.', the extension cannot be obtained correctly.

For example, [[File:Julia set (C = 0.285, 0.01).jpg]] is not recognized as a file because the extension is separated by '285, 0.01).jpg', and this problem is solved simply by replacing partition with rpartition.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (19b9ff2) to head (7c3cd92).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #129   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines         2234      2234           
=========================================
  Hits          2234      2234           

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[5j9]

Thank you! Merged via b6c7fc0 (I followed github command-line instructions to merge, don't know why it left this PR open). Released as v0.55.9.