Hi user64637,

We haven't receive any reports with lost attachments in customer files. Let's

1. Go to Gear icon and select Attachments. 
2. Look for the file name you've attached on the invoice. 
3. On the Links column, you'll see the customer name and amount and you can also click on the link so you'll see the transaction where you've added this attachment.

For your reference, you can also check this article: Add or delete attachments in QuickBooks Online.

Feel free to drop a comment below if you have any other questions, we're here to help.