Hello,
Here are some things to check when you get “You must upload a .wie file generated by this plugin” while attempting an import.
1. Make sure the file you are uploading has a name that ends with .wie.
2. Make sure the file is not empty (open it in a text editor).
3. Make sure you are using the latest version of the plugin (1.5.2) and WordPress (4.9.1).
4. Deactivate all plugins except Widget Importer & Exporter then try again in case a plugin is interfering.
5. If you’re on a multisite network, go to its settings and try adding “wie” to “Upload file types”.
Please let me know what results you get.
Were you able to get this working?
Closing due to inactivity. Feel free to reply if still seeing the problem.