export button: ignoring the extension for fasta sequence in proteins

duplicates issue #1827 on intermine, in case it is a imjs problem.
extension: the only option for the FASTA export format, is ignored for protein sequences (it works for dna ones).

Start by reproducing the export-button behavior with a protein sequence and FASTA format, comparing it with a DNA sequence and checking the requested extension. Trace the export path to find where the extension is applied, then confirm that protein FASTA exports preserve it; issue #1827 on intermine may provide related context.

Written by the indexing model from the issue text.