Please help us help you and fill where relevant:
Your LimeSurvey version: LimeSurvey Community Edition Version 6.5.16+240708
Own server or LimeSurvey hosting: Own Server
Survey theme/template: vanilla
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While working on a survey theme I noticed that the file_upload question is not keyboard accessible. This is because the button to select a file that is actually part of the modal doesn't do anything and instead an invisible file input seems to be positioned at the viewport position where the button is displayed. This might work for mouse clicks but for keyboard users the file input is in no way connected to the button. (It also lead to issues when I styled the button to a different size, because it no longer fully overlapped with the invisible file input.)

I am looking for some guidance on how to fix this issue. Is there a way to change the html of core question types? Or is there a simpler solution? Maybe some javascript to trigger the file input when the button is clicked? I'd like to change as little as possible.

Can you report this issue on our Bug Tracker ?
I check it's an old issue (exist in 3.X version) , I don't think you can fix it via template : github.com/LimeSurvey/LimeSurvey/blob/ma.../modal-container.php

It must be moved to a twig file before allow updating.

Maybe you can try some update here.