Force protocol relative (\ instead http:\ or https:\) for files added in the content editor (eg. post, page)
Before, when add media:
<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>
After, when add media:
<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>
The plugin works only for newly added media (does not change the older entries).