I'm going to import data from Excel book by right-clicking on table name and selecting "Import Data...".
And when selecting a file which has ".XLSX" extension (All letters are upper case), SQL Developer on Windows says the following error message:
File XXXX.XLSX cannot be opened due to the following error: null."
(When selecting a file which has ".xlsx" extension (All letters are lower case), this message is not said and can import from the file.