File > Open Genome from file... > Choose Reference File.
The allowed reference files extension are 2bit, bnib, fa, fas, fasta, fna, fsa, mpfa, fa.gz, fas.gz, fasta.gz, fna.gz, fsa.gz and mpfa.gz.
For windows it is working as expected by not allowing selection of any file other than above mention extensions.
But for MAC systems it is allowing selection file with any other extension as well which should not be allowed.
For ex. you can select "*.bed.gz" genome reference file from MAC users which is incorrect as its extension is not FASTA or 2BIT.