JIRA: https://jira.spring.io/browse/INT-3726
Since the `FileHeaders.FILENAME` header is relaxed String, we can use any value there.
For example some use-cases (e.g. unzipping) would like to use the relative path together with the file name
to restore the original directory structure in the target directory using `FileWritingMessageHandler`.
Use `resultFile.getParentFile().mkdirs()` in the `FileWritingMessageHandler` to recreate the directory hierarchy before the target file manipulating.
Add Documentation
Address PR comments
2be12160e9