Could you clarify what you mean by that? Was the file identified as a jpeg in Finder or as some other type? How exactly did you open it in Affinity?
Regarding opening saved jpeg files, if you right-click on one of them in Finder, what are the choices shown in the popup menu for "Open with..."? What app were you opening them with before? Is it still shown as a choice on the Open With menu?
You should be able to open jpegs with at least Preview or either of the Affinity apps using the right-click method, or by dragging & dropping the file on any app's icon that can open jpegs. To change the default app (the one that opens the file if you just double-click on it), in Finder do a 'Get Info' on one of your jpeg files, & in the "Open With:" section set it to whatever app you want from the pop-up list. Then click the "Change All" button to apply that to all files of that type.
This sounds like a separate issue. What happens when you try?
