Why would you send files from your portfolio instead of adding them to your profile in the portfolio section?
1. make sure they have a verified payment source
2. don't start work until 100% is in escrow ( ideally have released 50% upfront )
3. make sure you send terms of service BEFORE you begin. So they cant say they didn't agree to anything.
4. Do no, do not, do not give them ANY mock work. I.e Can you write 10 headlines so i can see if you can do it.
They should be able to tell from your portfolio that you can do the job, otherwise they are trying to con you into giving them free work
Don't agree to do work outside of the upwork site, no matter what they say, sob story they give, etc. Anything that needs to be handled should be done onsite. Except apparently the message system here doesnt always work, so you might need to communicate via your email.
Even when I used to communicate via email offsite, I would copy the emails and post them to the Messages section, so there was always a record of the message with the job (this was on elance...messages don't work the same way here, but a record of them here might be helpful).