Hi, Andrew
Welcome to Microsoft Community.
We understand that you have a problem searching for files in the file explorer. I tested according to the steps you provided, and found that I will encounter the same problem as you. This is essentially because the file resource manager adopts the logic of fuzzy query, but it can't retrieve irregular file types, such as custom file types.To solve this problem, there is an alternative at present, that is, when you search for files by keywords, first enter the complete keywords, then filter the file type, then delete the file suffix of keywords in the search bar, and then filter the file type again, so that you can select the files you searched.**For example, if you want to search JAN.outlook, you can enter "JAN.outlook" in the search bar completely, and then follow the steps in your GIF to filter the file type. You will find that you can't select the file. At this time, change the keyword to "JAN" in the search bar, and then re-filter the file type, so that you can successfully select the "JAN.outlook" file you want.**You can give feedback on this issue through Feedback Hub, and the Microsoft development team will review the proposal of this issue and gradually improve and enhance the user experience in subsequent versions. You can share the feedback link with us after submitting your feedback, and we will vote for you.If you have any questions, you are welcome to visit us at any time.
Best Regards
Nicholas.Z - MSFT | Microsoft Community Support Specialist