You can share a file with the geek within the messaging area on your dashboard.
Visit the Purchases tab on your dashboard and click Files on the service related to the file you want to share.
You should see a Files section open on the right. Click the blue +Add New button in that section to upload a new file.
Once the file has been uploaded, you should see it appear in the Files section.
If you click on Messages, you should also see a message was automatically sent to the worker, giving them a link to download the file.