Can't upload PDF to panel - error: 'You are not allowed to upload invisible files'

HI!
I have a field of uploading PDFs to the panel. Sometimes it works but for some of the PDFs that I try to upload it gives me an error: “You are not allowed to upload invisible files” .

Any idea why it happens?
Thank you!

Invisible files means that your file names are starting with a dot. These are not allowed. Normal files shouldn’t start with a dot, they are hidden in the file system.

you’re right! thanks