At 18MB the file is just a little over the default maximum filesize for sending to avast.

Open avast → settings → Virus Chest → change the “maximum size of file to send to avast” to 20000KB (20MB)

You will then be able to send to avast.

EDIT: It will take a while, it is with me…maybe sending in a password protected archive to virus(at)avast.com would be better

Scott