Fix Apache bug flag handling

[akshaysharmajs]

Will only set check_for_apache flag only if supplied_type belongs to one of text/plain, text/plain; charset=ISO-8859-1, text/plain; charset=iso-8859-1, text/plain; charset=UTF-8

Fixes #13

[codecov]

Codecov Report

Merging #14 (b54bb0c) into main (fa67530) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #14   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          352       352           
=========================================
  Hits           352       352           

Impacted Files	Coverage Δ
xtractmime/__init__.py	100.00% <100.00%> (ø)

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