ClamAV

An open-source antivirus engine that scans files and mail attachments for known malware using signature databases from the ClamAV project. It can run as a background daemon behind a socket, as a command line scanner for scheduled jobs, or as a filter that inspects uploads, while a separate updater refreshes the signature databases on a schedule.

It is commonly used to screen web application uploads, filter inbound and outbound mail, and scan shared folders.

Hosting the scanner yourself means sample files and signature state stay on machines you control, there is no charge per scanned gigabyte, and recovery after an incident means restoring your own configuration and databases.