thumbnail: KIO::filesize_t type for sizes instead of qint64_t
It will fix overflow for m_maxFileSize from default max value
for KIO::filesize_t (that is quint64_t, not qint64_t)
after commit: 04a1a976.
Also this change will remove the difference and will make handling file sizes similar to how it is done in KIO -> PreviewJob (there is KIO::filesize_t for size).