Hi NewPowerUser_Latam,
I apologize for all the trouble it has caused and sorry about your time.
As per your description, generally in SharePoint online we select the item attachment icon that we want to open attachments, it will open directly in browser.
Note: Online editing is only applying for Office files attachment.
As a workaround is saving the attachment in a library and adding URL rather than attachment to the list. Or create a Hyperlink column in SharePoint online and paste a URL in that column to directly open in browser.
I appreciate your patience and understanding.
Best Regards
Waqas Muhamma