RecentUsageModel: propagate mimeType to KFileItem ctor

BUG: 401579
FIXED-IN: 5.25

Best used in conjunction with with
frameworks/kio!780 which will
allow kicker recentUsageModel not to make any stat call but solely
base its iconName based on passed mimetype, allowing to act as if
KFileItem::SkipMimeTypeFromContent was set.