title: Use Paperclip validations without storing the attachment
body: <p>I am building a Rails endpoint that proxies a different service and maps the responses from said service. The main problem is to pass along a file attachment's byte data to that service.</p>
  
  <p>A constraint is that I must do some sanity checks on the file before I pass it along.</p>
  
  <p>I have no need to persist the file in my Rails app, it is merely used as input to the other service.</p>
  
  <p>In a very simple implementation I simply just read the bytes from the appropriate request parameter which is wrapped in a <code>Tempfile</code>, but this of course entails no sanity checks, and therefore is not good enough.</p>
  
  <p>I am interested in doing the kinds of validation that Paperclip supports, in particular size and content type, but I would prefer to not store the actual file anywhere.</p>
  
  <p>Is it possible to use only the validation parts of Paperclip and not store the attachment anywhere?</p>