The only way that I know of to view the message is:
- Select the Files tab at the top of the screen
- Select Sent Files on the left
- Check the box next to the file(s) that you sent
- Select Forward at the top
At this point, the Subject & Message field on the next tab will be filled in with your previous Subject and Message.
If, in general, having an easily-retrievable record of what you type in is important to you, you can include yourself as a Recipient on the transaction.
Hope this is helpful.