Paperclip client-side validation example

Like the thoughtbots paperclip gem and DockYard client_side_validations gem.

It’s strange, but till now I could not find any example of client-side validation for paperclip attachments, except for presence validation.

Freshly added example for content-type validation and file-size validation to the client-side validation wiki. Soon I’ll create a pull request for it or a standalone gem if PR would be rejected.