Thank you for using my plugin!
Please try to set the following value on the setting menu.
json = application/json
If you want to pass the mime type check by WordPress, you had better set the value is the follows;
json = text/plain
If you want to know the value of the mime type by WordPress;
For macOS
1. Download check_finfo_file.php (https://github.com/kimipooh/wp-add-mime-types/blob/master/tools/check_finfo_file.php) to the Downloads folder.
2. Open Terminal app.
3. cd $HOME/Downloads
4. php check_finfo_file.php URL or File_path.
