You can use whatever means you and the client want to communicate and to send files, etc. After a project starts, I rarely use UW for anything. Most of my communications is via Slack and Trello. This is not an issue.
To clarify what was stated above about fixed price protection. The only protection offered is the escrow service you are already participating in. Fixed projects don't get the same payment protection that time based contracts do. You should absolutely submit the milestone of course in order to get the review process going in order for escrow release. However, files do not need to be part of that from an Upwork perspective.
I will share something I learned the hard way.
Clients should not obtain any hard copy of a file while the project is ongoing.
Therefore you should just make one drive account. 10 EU per month but when you share a file you have the option to give just VIEW PERMISSION ONLY. They can comment on it, ask for changes and etc but you and only you have the rights to the link. Like this, you protect yourself from cheap ass people who are ready to stab you and go to a cheaper monkey like freelancer.