This file is indexed.

/etc/Muttrc.d/charset.rc is in mutt 1.7.2-1+deb9u1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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# Some GB18030 traditional Chinese mails are wrongly labelled GB2312.
# The first charset is a superset of the second. Let's alias it, so
# that Mutt displays such mails as if they were correctly labelled.
charset-hook ^gb2312$ gb18030

# Some mailers send EUC-JP-MS Japanese mails wrongly labelled EUC-JP.
# The first charset is a superset of the second. Let's also alias it.
charset-hook ^euc-jp$ euc-jp-ms