Update MimeTypeExtensionGuesser.php

Add ```'text/rtf' => 'rtf'``` type, as it is a valid type specification for rtf documents.
minor #11128 Update MimeTypeExtensionGuesser.php (kevinklika)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #11128).

Discussion
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Update MimeTypeExtensionGuesser.php

Add ```'text/rtf' => 'rtf'``` type, as it is a valid type specification for rtf documents.

Commits
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c986b91 Update MimeTypeExtensionGuesser.php