You can send files to a client any way you want to.
Or any way the client wants you to.
FTP. Dropbox. Email. USB flash drive. Google Drive. Microsoft One Drive.
It's up to you and the client.
Many freelancers like to send files using the Upwork Messages tool and/or "Submit Work / Request Payment" button, even if they send files using another method.
If you have a fixed-price contract, you should always click the "Submit Work / Request Payment" button - even if you don't actually submit files using that button, and even if you have provided files through some other method. Always click that button, so that you can start the countdown clock for automatic payment.
I checked your account and can confirm that you do not have any active contracts on Upwork. I would recommend that you do not submit the work until the client sends you a fixed-price offer. Once you accept the offer, this should show up on your My Contracts page as an active contract. Make sure that the client's payment method is verified before submitting the work. Here is more information on how you can get paid for fixed-price contracts.
I highly recommend that you read up the "Safety First!" section of the freelancer resources we have compiled, and these tips for avoiding questionable jobs for more information about working safely through Upwork.