Good evening, @Roland12. 

I appreciate you coming here with your question about attachments in QuickBooks Online. 

To clarify, have you reviewed all the supported details of how the attachments should be entered/what should be? Here's a guide if not: 

Add or delete attachments

If so and none of these explanations cause the problem you're having, then I recommend clearing your browser's history. This should eliminate any errors or glitches that could be happening in your account. 

Come back if your issue persists. It's my priority that you're able to bring the attachment up into a new tab rather than downloading it. I'll be waiting for your response!