Fixes to neglected Mobile upload interface (for 3rd parties)

Adding mw-ui-input class to description field as this is now needed
for textareas to be properly styled.

Moving 'text-align: center' up one element so that the upload button is
also centered (like all the other content on the page).

Bug: T74972
Change-Id: I1c1164002625ea6a933bb913474d5d86e5a1523c