RFR: 8260020: WebView does not render contents of locally stored MarkDown (*.md) files.

[Gopal Pattnaik]

Local markdown file mime type support was missing in open source [JDK-8260020],

Hence, we added a mime type for local Markdown files display.

expected bahaviour: This should display the markdown file content as text in the webview.
Verification:
The test passes with the rendering of local Markdown file.

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Commit messages:
 - JDK-8260020 Local Markdown files not shown in Webview

Changes: https://git.openjdk.org/jfx/pull/1788/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1788&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8260020
  Stats: 6 lines in 2 files changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1788.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1788/head:pull/1788

PR: https://git.openjdk.org/jfx/pull/1788