The TallyPage has two forms, built in TallyPage::showLocalForm and TallyPage::showUploadForm. Use OOUI for both of these.
|T271355 Improve Tally functionality
|T271830 Allow election decryption key to be submitted via the TallyPage form
|T272716 Convert SecurePoll TallyPage to use OOUI
|T243476 Create an HTMLSelectFileField
|T170263 Create a PHP fallback for SelectFileWidget
For reviewing/testing: both modes of the tally form can be seen by tallying an encrypted election.
The dump file can be created by clicking "Dump" on Special:SecurePoll.