Unfortunately, it isn't possible to upload certain files, such as ads.txt, via FTP.
Luckily, however, this won't be a problem when it comes to your Google AdSense activities. According to Google:
Google uses the content of any ads.txt files hosted on a root domain to determine which seller accounts are allowed to serve ads on that domain. Google runs an auction and returns a winning ad for requests on sites where an ads.txt file exists with a correctly listed publisher identifier. If the identifier in the file is incorrect, an auction is not run for that request.
Webador ensures that no ads.txt file can be found at your domain name, which means all your ads will be approved, and you don't have to worry about loss of income.
✔ Tip: Check out this article for more information regarding FTP or PHP.