When you need a File object but only want part of the file accessible, there is FileSlice

file_slice.rb

class FileSlice < File

  def limit(lower,upper)
    @lower = lower
    @upper = upper
    @offset = @lower  
    rewind
  end

  def rewind
    seek(@lower)
  end

  def size
    (@upper ||= super) - (@lower ||= 0)
  end

  def read(length = nil)
    return nil unless @offset < @upper
    maxread = @upper - @offset
    data = super(length ? [length,maxread].min : maxread)
    @offset = pos
    data
  end

end