QGIS » QGIS Application

The code differences are:

r.out.bin:

75         parm.output = G_define_option();
76         parm.output->key = "output";
77         parm.output->type = TYPE_STRING;
78         parm.output->required = NO;
79         parm.output->description =
80             _("Name for output binary map (use output=- for stdout)");

but r.out.tiff:

106         outopt = G_define_option();
107         outopt->key = "output";
108         outopt->type = TYPE_STRING;
109         outopt->required = YES;
110         outopt->gisprompt = "new_file,tiff,tiff";
111         outopt->description = _("Name for new TIFF file");

Apparently the presence of gisprompt disactivates the directory browse button in QGIS.