A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Changal Rayudu,
Thanks for writing back and updating with us.
As per the further updates, I am sorry to convey that the attachments added in the SharePoint Online List item will be directly opened in the browser and it can’t be previewed because the attachment added to the item will be added as the URL, where opening the URL can’t be previewed and it can be only accessed in the browser.
I agree with you having the open the attachments in the List item as preview instead of opening in the browser in the SharePoint Online will benefit the users and I suggest you to add your valuable idea as feedback in the SharePoint · Community (microsoft.com) which is the best place to share our ideas and improve the Microsoft products.
Appreciate your patience with us and have a wonderful day!!
Best Regards,
Sophia