Improve "select document format" dialog to allow user to choose any format

The sequence below is in GFF format. Even if I create a file with .gff extention the sequence is not recognized as a GFF file.

In "Select document format" along with advised formats introduce a combo box with all supported formats. It allows user to select any format if a format he needs is not in the advised list for some reasons.

#LINK
#lang en
#attribution_url http://genocon.org
#attribution_name Cox
#file_name camv_scrambled1_sequence
#download_from http://linkdata.org/work/rdf1s141i
#property http://purl.obolibrary.org/obo/sequence_of
#object_type_xsd string
#property_context Assertion
camv_scrambled1_sequence GTCAACATGGTGGAGCACGACACTCTCGTCTACTCCAAGAATATCAAAGATACAGTCTCAGAAGACCAGAGGGCTATTGAGACTTTTCAACAAAGTTAATGCGAGGGAAACCTCCTCGGATTCCATTGCCCAGCTATCTGTCACTTCATCGAAAGGACAAGTGAAAGAGAAGATGGCTTCTACAAATGCCATCATTGCGATAAAGGAAAGGCTATCGTTCAAGATGCCTCTACCGACAGTGGTCCCAAAGATGGACCCCCACCCACGAGGAACATCTAGGGGAAAAAAGACGTTCCAACCACGTCTTCAAAGCAAGTGATTGGATTAAGGTTCTTCCACACGGTAAGGGATGGCACTAACACCTACCATCCTTCGCAAGACCCTTCCTCTATATAAGGAAGTTCATTTCATTTGGAGAGGACCTCGAC