Very simple example of accepting an XHR upload in Sinatra

upload.rb

post '/upload' do                                              
   # This accepts the XHR uploads and save the file into the script home directory
   
   string_io = request.body # will return a StringIO
   
   data_bytes = string_io.read # read the stream as bytes
   
   filename = env['HTTP_X_FILE_NAME'] # This will be the actual filename of the uploaded file
   
   # Write it to disk...
   File.open(filename, 'w') {|f| f.write(data_bytes) }                                                      
end