Access Denied After Uploading a document

Symptoms: A user uploads a document and receives an access denied page. The document is not uploaded.

A few assumptions:
1) you use ForeFront antivirus for SharePoint
2) a user uploads a document that is blocked by antivirus (most common would be a zip file with an uncompressed file size above 100mb)
3) the document library has unique permissions and the user has at least contribute rights, but has either Limited Access or Read access to the parent site.

What's happening is that the user is getting redirected to /_layouts/avreport.aspx because forefront is blocking the upload, and the user gets denied access on avreport.aspx (you'll notice this if you look in either the IIS logs or the query string of the access denied page). This is because a user must have at least contribute rights to the site to have access to avreport.aspx.