Merge branch 'attach-file-cursor-position' into 'master'

Adds the markdown link at the cursor poisition

When writing a comment and then dropping an image it will correctly add the image markdown link at the cursor position

![drop-image-markdown](/uploads/52f41e38faca48c9ee01a02a75ec71be/drop-image-markdown.gif)

Tested in Chrome & Firefox and both working fine  

Fixes #12822 

cc. @jschatz1 

See merge request !2648