Little help WordPress Format Gellery Item

Hi all,
Can someone help me with something?
I test my theme that i want to upload here on WordPress, i imported dummy data, and in post Post Format: Gallery, when i click on image, will open in new page. This page i want to build new template.

Which is the name of file that i need to create it to modify template ?

I tried with all from body class to name the php file but nothing.

My body class=“attachment attachment-template-default single single-attachment postid-616 attachmentid-616 attachment-jpeg logged-in admin-bar customize-support”

PS: i want to build new template because this page not shown my sidebar.

try create “image.php”

Yes, i forgot that i made image.php and before your msg i just finish.

Thanks also for good idea.