D13898: Use non deprecated fastInsert in file.cpp (first of many to come)

[David Faure]

dfaure accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R241 KIO

REVISION DETAIL
  https://phabricator.kde.org/D13898

To: jtamate, dfaure, #frameworks
Cc: kde-frameworks-devel, michaelh, ngraham, bruns