Upload: Better align file input and buttons in plugin/theme upload form.

Follow-up to [47271].

Props seedsca, piyushmca.
Fixes #49914.
Built from https://develop.svn.wordpress.org/trunk@47599


git-svn-id: https://core.svn.wordpress.org/trunk@47374 1a063a9b-81f0-0310-95a4-ce76da25c4cd