You can export the CSV as a download file by encoding the CSV string and using the loading methods for the anchor createElement tag object. See the code below, which is a small modification of your implementation.
jsFiddle, FusionCharts V 3.3.1
var exportMyChart = function (type) {
var chartObj = FusionCharts('myChartIdAmount4');
if (chartObj.hasRendered()) {
if(type === 'CSV'){
var a = document.createElement('a');
a.href = 'data:attachment/csv,' + encodeURIComponent(chartObj.getDataAsCSV());
a.target = '_blank';
a.download = 'export.csv';
document.body.appendChild(a);
a.click();
}
else{
chartObj.exportChart({ exportAtClient: '1', exportFormat: type, exportAction: 'download' });
}
}
else{
alert("What are you trying to export?");
}
}