RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v17]

[Brian Burkhalter]

> Resurrection of the proposal to add a method to obtain the filename extension originated in PR [2319](https://github.com/openjdk/jdk/pull/2319).

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8057113: Change specification as suggested in review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/8066/files
  - new: https://git.openjdk.org/jdk/pull/8066/files/8a4a4bc6..d8df5141

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=16
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=15-16

  Stats: 10 lines in 1 file changed: 2 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/8066.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/8066/head:pull/8066

PR: https://git.openjdk.org/jdk/pull/8066