INSTALL.rst not rendered as RestructuredText

[Ryan J Ollos]

One of the tasks in #12120 is to add the rst extension to INSTALL and UPGRADE. However, INSTALL.rst is rendered in plain text due to the default value of [mimeview] mime_map_patterns.

[mimeviewer]

mime_map_patterns

List of additional MIME types associated to filename patterns. Mappings are comma-separated, and each mapping consists of a MIME type and a Python regexp used for matching filenames, separated by a colon (":"). (since 1.0)

text/plain:README(?!\.rst)|INSTALL(?!\.rst)|COPYING.*

Another issue is that both UPGRADE.rst and INSTALL.rst (after correcting above issue) are rendered with mimetype text/prs.fallenstein.rst. The issue may only occur when Pygments is installed. More investigation is needed.