Add dita file extention to the XML type

2nd try. Add dita file extention to the XML markup.
DITA is the OASIS Darwin Information Typing Architecture used for technical documentation.
@see https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=dita
Merge pull request #357 from uo-hrsys/patch-2

Add dita file extention to the XML type