attach file open new tab

This is not a critical task.

Although it works in content edit page where clicking on attachment opens it in new window, but not so in case of view.

Have you checked the permissions?

There are two ways to implement the changes for attached file in content view by adding the below code before return in file/file.module(function theme_file_link() ) :

1) To open file in new tab
$options['attributes']['target'] = '_blank';

2)To open file in new window

$options['attributes']['target'] = '_blank';
$options['attributes']['onclick'] = "window.open(this.href, 'filePreview','toolbar=0, scrollbars=1, location=1, statusbar=1, menubar=0,resizable=1,width=500,height=550');return false;";

Add the above code under Drupal Core module Path:"[Your Drupal directory]/modules/file/file.module" at Line : 746, function:theme_file_link() before "return".

Hello

Thank You monish_deb for the solution.
You saved my days.

Shraddha
Carving IT Pvt. Ltd.

will have to add patch for drupal 8.8.x as well. This is not working drupal 8 as well.

Adding patch for 8.8.x