Update MimeTypeExtensionGuesser.php

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

Author: kevinklika

File/MimeType/MimeTypeExtensionGuesser.php

@@ -723,6 +723,7 @@ class MimeTypeExtensionGuesser implements ExtensionGuesserInterface
         'text/plain' => 'txt',
         'text/prs.lines.tag' => 'dsc',
         'text/richtext' => 'rtx',
+        'text/rtf' => 'rtf',
         'text/sgml' => 'sgml',
         'text/tab-separated-values' => 'tsv',
         'text/troff' => 't',