The Calendar Import view does not allow to drag and drop plain text files that do not have the extension .json. There is no hint for the allowed extensions. The error message is xxxExtension is not allowed. When opening the file select dialog the dialog is not initialized with the .json extension.
The user have no chance to get the rule for file import.
The calendar import dialog should reflect the allowed extensions in the error message. The allowed extensions should be should in the drop box like Drop Calendar .json files to upload. The open file dialog should show only the files with the allowed extensions.