I found myself in several situations where:
Just use the form at the top of this page. Is as simple as that. After you submit, we'll inline it for you.
Maybe you want to post to inlineit.heroku.com/work with :html and :css ?
Me Ivan Acosta-Rubio did it while working for Bakedweb - A design and web development studio in Miami Florida.
The real credit for this it to the creators of the Tamtam gem to Sinatra and Benlovell in #sinatra IRC for pointing in the right direction of Testing File uploads on Rack
The github repository is here.
The document you are reading is much longer than the app. =)