File upload - Possible to change Choose File on mobile | How to act on event that file is chosen?

The context is a non-native mobile app to upload videos. After the anonymous user specified his name and email address, he must choose a file to upload and upload this. I’ve basically two questions related to this scenario: On a normal page, I can change the wording of the button for uploading the file. Mobile, I didn’t find how to do that. It is “Choose File” and there is no way for me to change that. I guess I could get there using css. Am I correct you cannot do this for mobile? Is that device determining what the widget shows? After the user pushed Choose File and has chosen the desired file from the phone’s folders, the page is showing the name of the chosen file. From what I could see, I think the client is unaware that anything happened (although the chosen filename is shown). After refreshing the page’s file object, the file object does have content. My question, is there anyway for me to find out that a file has been chosen without refreshing the file object? This would allow me to show the button to Save the file object only if a file has been chosen. Ideally, I would not bother the user with having to Save the file object but save it automatically after the file is chosen. I did not find a way to do that either...    
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