Uploaded pdf file with multiple dots (.) in name are renamed for security reason
when the upload file name contains multiple (.) in file name, after each dot it will be considered as extension of the file. In your case(this.is.an.example.10.pdf) it will check (is,an,examplem,10) in valid extension, It is not match with that it will add (_)inthat. If your uploaded file name is not same after drupal changes it will give that error. For more detail check file_munge_filename