Not able to open file using 'FileOpener'

This is my code to open file. No errors and its printing “File is opened” success message but file is not opened. Am i missing something?

    let targetPath = this.file.externalRootDirectory + '/Download/' + 'sample.pdf', 'application/pdf')
    .then(() => console.log('File is opened'))
    .catch(e => console.log('Error opening file', e));  

Remove the / before Download and try. You can also do console.log(targetPath); to see what you are getting.