You can use AxCrypt app to secure your asset files. So without the file password, nobody can open the encrypted files. Whenever you want to see that file, just decrypt the same using the AxCrypt app.
Please note: AxCrypt encrypted files should have an .axx file extension and it will be opened and decrypted only with the AxCrypt app.
If the above does not help, Sorry to say we didn’t get the exact issue. So please provide more information about the issue and write a mail to firstname.lastname@example.org .
If you can send a screenshot showing where the problem is, it often helps us understand.