On mobile, file actions were being displayed mostly off screen, to the

right or left of the file attachment thumbnail. This change fixes things

by displaying the file actions below the file thumbnail. We also make

sure to hide the actions if the user opens the caption editor.

Tested using file actions on mobile and desktop.
Tested opening the caption editor while file actions are open.
Tested tapping and hovering on multiple different thumbnails, opening

  and closing file actions for each.