Fix plain text Note document preview displaying as markdown

Note documents with note:mime_type 'text/plain' display in nuxeo-document-preview as markdown.

Steps to reproduce:

1. In Web UI, create a Note document of mime-type 'Text'.
2. Edit Note content to contain markdown formatting - for example:

# Title
---
- item 1
- item 2
- item 3

Expected behavior: Note document preview displays as plain text (pound and dash characters visible; rendered as typed)

Actual behavior: Note document preview displays as markdown (Title text bolded, bullet points ahead of item text, and line separator between both).