The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
The source tarball contains a prebuilt file in the Shockwave Flash (SWF) or Flash Video (FLV) format. These are often included by mistake when developers generate a tarball without cleaning the source directory first. An exception is simple video files, which are their own source.
If there is no sign this was intended, consider reporting it as an upstream bug.
If the Flash file is not meant to be modified directly, please make sure the package includes the source for the file and that the packaging rebuilds it.