RFR: 8293971: Loading new Media from resources can sometimes fail when loading from FXML

[Alexander Matveev]

There was two problems:
 - uri.getPath() was returning null for jar URI, since it is not a standard URI and thus we did not detect file type based on extension.
 - Our signature detection for MP3 had a bug and did not detect MP3 correctly. See comments in code.
 
Fixed by adding function to extract file name from jar URI and also signature detection was fixed for MP3.

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Commit messages:
 - 8293971: Loading new Media from resources can sometimes fail when loading from FXML

Changes: https://git.openjdk.org/jfx/pull/902/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=902&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293971
  Stats: 51 lines in 2 files changed: 41 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jfx/pull/902.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/902/head:pull/902

PR: https://git.openjdk.org/jfx/pull/902