Hi @Zane Richards
Welcome to our forum!
According to your description, i reproduce your issue. Based on my research, it is not yet supported to open an attachment file with its default formatting application in New Outlook. I am afraid that you need to switch back to the legacy Outlook to continue to use this feature.
Please kindly understand that many functions are not designed at present. To get this feature more quickly, I have found a post in feedback collection website, and you could vote for it. Many features of our current products are designed and upgraded based on customers’ feedback. You can also make suggestions on that website and hope Microsoft will notice this in the future.
Thanks for your support and understanding!
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our [documentation] to enable e-mail notifications if you want to receive the related email notification for this thread.