You can upload and send files through the Upwork Messages, but the problem is that you were not hired for this job on Upwork. I see the job entails getting paid directly, which is a serious violation of Upwork ToS. You are required to get paid through Upwork for jobs and relationships you acquire through Upwork. Please stop communicating with this client and report them directly to Customer Support.
If you go to Find Work > Job Applications you will be able to check whether any of your applications were accepted. Applications you see in the "Active Candidacies" section have been accepted and the client would like to discuss the job further or have an interview with you, but that doesn't mean the job is awarded to you.
You should definitely not do any work until you've been awarded the job, which would show in your "My Jobs" tab. You can find more details here.
I strongly advise you read the articles from the Getting Started as a Freelancer section of the Hep Center before you proceed with sending new proposals. I also advise watching our educational videos and reading the best practice advice shared by other users:
To help you work safely on Upwork, please have a look at tips and warning signs shared in this thread.
I checked this for you and our team has reached out to you already regarding this concern. Please check this link for the ticket related to your concern or check the last email sent to you by our team. Kindly follow up with our agent and they'll assist you accordingly.