Downloaded PDF and Excel files are corrupted

I have to download files(for now only pdf /xlsx/xls) which I recieve as byte array (charset utf-8)
I saved the respective extensions in the mimetypes :
const mimeTypes = {
pdf: 'application/pdf',
xls: 'application/vnd.ms-excel;charset=utf-8',
xlsx: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet;charset=utf-8'
};
and my saveAs function:
saveAs(new Blob([data], {type: mimeTypes[ext]}) , `${fileName}.${ext}`);

All files are getting downloaded but they are all corrupted.