What? There are plenty of things you can charge extra for:

-direct linking/embedding (also solves your ad problem)

-increased upload/download speeds

-data center replication

-custom branding

-custom domain

-increased storage or bandwidth

-increased time before file/account expiration

-increased file types or sizes

Have you done any research into this space in terms of what’s feasible to offer for free? If you had, you’d see plenty of space for paid upgrades, like the above.