maximumContentLength can do IO

[geekingfrog]

The current type signature for maximumContentLength is site -> Maybe (Route site) -> Maybe Word64.
I'd like to have the ability to restrict the content length based on a path parameter of the route, so I would need a db lookup.
For example: /upload/foo -> 2MB max, but /upload/bar -> 10MB max, these mappings are stored in a db.
Is there any workaround I can use for now? Is it something which could be added to the next release?

[snoyberg]

We had a similar situation with shouldLog in the past, and added a shouldLogIO method to the Yesod type class to address this. I think we could do the same thing in this case. Would you be interested in taking a stab at such a PR?

For comparison, here's the prior art: aa5781d