How to create an image file from Firebird blob data?

I have a Firebird DB table, which contains fields with image data (as blob sybtype -5). I need to get this data and convert it to an image file. Code example below creates a file (but size is very large, and while trying to open it – it is said, that file is not right BMP file). What am I missing here?

result[0].OBJ_IMAGE1((err, name, eventEmitter) => {
    if(err) throw err;

    let chunks = [];
    eventEmitter.on('data', chunk => {
        chunks.push(chunk);                
    });
    eventEmitter.once('end', () => {
        let buffer = Buffer.concat(chunks);         

        fs.writeFile('test.png',  buffer, err => {
            if(err) throw err;
            console.log('image file written');
        });            
    });
});