Upload: Improve file input and button alignment in plugin/theme upload form.

Resets flexbox on smaller screens. Improves vertical alignment on large screens.
Follow-up to [47599] and [47271].

Props mukesh27, DarkoG.
See #48859, #49914.
Fixes #49951.

Built from https://develop.svn.wordpress.org/trunk@47750


git-svn-id: https://core.svn.wordpress.org/trunk@47526 1a063a9b-81f0-0310-95a4-ce76da25c4cd