<a id="bmpFormat">.BMP</a>

<script>
    var exportBMPElement = document.getElementById('bmpFormat');

    exportBMPElement.addEventListener('click', function(e) {
      map.once('postcompose', function(event) {
        var canvas = event.context.canvas;
        exportBMPElement.download = 'mapa.bmp'
        exportBMPElement.href = canvas.toDataURL('image/bmp');
      });
    }
</script>

<div class="modal>
  <form id="exportMapForm" action="javascript:doExportMap()" role="form">
  </form>
</div>

function doExportMap() {
    var a = document.createElement('a');
    a.download = 'tux.png';
    a.href = 'http://mail.rsgc.on.ca/~ldevir/ICS3U/Chapter4/Images/tux.png';
    a.click();
}

document.getElementById("exportMapForm").appendChild(a);

function doExportMap() {
    var a = document.createElement('a');
    a.download = 'tux.png';
    a.href = 'http://mail.rsgc.on.ca/~ldevir/ICS3U/Chapter4/Images/tux.png';
    document.body.appendChild(a);
    a.click();
    document.body.removeChild(a);
}