[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").