D26407: KFileItem: improve isSlow to never block, make SkipMimeTypeFromContent skip only network fs

[Méven Car]

meven marked 3 inline comments as done.

REPOSITORY
  R241 KIO

REVISION DETAIL
  https://phabricator.kde.org/D26407

To: meven, #frameworks, ngraham, broulik, dfaure
Cc: anthonyfieroni, kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, 
bruns