Micrososft has released a security update for attchments within Outlook that checks for informalities. Outlook shows that attachment is potentially unsafe when the filename contains ".." (or mulitple dots) before the file extension.
If the company name within QuickBooks has a "." at the end it will attach the attachment with two dots. You will have to remove the dot at the end of the company name to stop this from occuring. The attachments will still send, but some receiving end servers will strip the attachment from the email before it reaches its intended destination.
From the screenshot above you can see that the company name is "Test123." and the attachment has two dots. When the Send function is clicked the above error message will appear.