Fastmover

A simple drop down selection meta box on a post edit page will allow you to specify default, HTTP or HTTPS. Default will allow both HTTP and HTTPS protocols.

Protocol Enforcer

This plugin allows a user to specify a post to only be viewed over HTTP or HTTPS.

Patryk

Force protocol relative (\ instead http:\ or https:\) for files added in the content editor (eg. post, page)
Before, when add media:
<pre><code><a href="https://wordpress.org/wp-content/uploads/some-cool-photo.png"><img src="https://wordpress.org/wp-content/uploads/some-cool-photo.png" alt="Some cool photo" width="1920" height="1410" class="aligncenter size-full wp-image-19200813" /></a>
</code></pre>
After, when add media:
<pre><code><a href="//wordpress.org/wp-content/uploads/some-cool-photo.png"><img src="https://wordpress.org/wp-content/uploads/some-cool-photo.png" alt="Some cool photo" width="1920" height="1410" class="aligncenter size-full wp-image-19200813" /></a>
</code></pre>
The plugin works only for newly added media (does not change the older entries).

PRyC WP: Force protocol relative to uploaded media

Force protocol relative (\ instead http:\ or https:\) for files added in the content editor (eg. post, page)

Protocol Enforcer vs PRyC WP: Force protocol relative to uploaded media