Digging out this issue as I experience the same.
What does not work: drag & drop a .svs file while having the correct Bio-Formats configuration (see snapshot)
What does work: Plugins > Bio-Formats > Bio-Formats Importer > Pick your .svs
Maybe it’s more a general issue with Bio-Formats and some formats, so have you experienced the same with .svs or other formats?
Would be nice to report this to Bio-Formats’ team all pulled together…