Fix MIME type not determined for small files

Summary: Previously, LoadingDocumentImplPrivate::determineKind() was never called for files < HEADER_SIZE when transferred via KIO, which prevented to load them correctly.

Test Plan: Modify LoadingDocumentImpl::init() to make it always load files via KIO. Compile and execute documenttest. Before the fix it will be stuck in an endless loop. Afterwards it completes normally.

Reviewers: Gwenview, ngraham

Reviewed By: Gwenview, ngraham

Subscribers: ngraham, Gwenview

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D22305