[Thumbnail] Fix lookup of legacy thumbnailer plugins

For most legacy plugins, "filename" and plugin id are the same, but
the marble plugin "marblethumbnail" provides e.g. marble_thumbnail_kml.

The KIO previewjob.cpp uses the filename:
`job->addMetaData(QStringLiteral("plugin"), currentItem.plugin.fileName());`

Regression from commit 69dc61d7 ("Utilize
KIO::PreviewJob::availableThumbnailerPlugins").


(cherry picked from commit e68a2f24)