I have not found a simple default setting for new files in the Windows application. There is no way to check for Mac.
You can try to log out of the application through the menu ⟩ Log Out. This should clear the cache, cookies, and other app data.
You can also find the “settings.json” file, open it in a text editor and change the value of the parameter “defaultEditorType” to “design” so that the Design file is created by default, or just leave the quotes to reset the default settings. It should look like this: "defaultEditorType": "".
Figma Beta path for Windows: C:\Users\%USERNAME%\AppData\Roaming\Figma\DesktopBeta\settings.json