How big is the file exactly ?
The VT limit is 10MB.
If you have it in the chest, open the chest and right click on it and select email to Alwil software (I know, I know), it doesn't actually get sent by email if you are on the latest versions of avast (latest is 4,8.1296), Fill in the pop-up form Type: possible false positive (see image 1) and submit.
This is uploaded during the next avast manual or auto update process, after the update is doenloaded and before it is installed the sample will be uploaded, see image 2.