Spring Refactor: Replace hardcoded '.' with EXTENSION_SEPARATOR in StringUtils

This pull request refactors the StringUtils class by substituting all occurrences of the hardcoded '.' character with the already-defined constant EXTENSION_SEPARATOR. This refactor promotes cleaner code by removing magic values and enhancing maintainability, as separator characters are now managed centrally. Additionally, this change minimizes the risk of errors that may arise from inconsistent use of the '.' character throughout the codebase.

No changes have been made to the functionality of the methods, ensuring that the behavior remains intact.

Comment From: bclozel

Thanks but in both cases the "." characters are not extension separators but have other meaning.

Comment From: SungbinYang

Thank you for your comment.