asset.rb

# app/model/
# Your model where you've defined has_attached_file
# Do not define :original in your styles
class Asset < ActiveRecord::Base

  attr_accessible :position, :picture_cover, :asset
  belongs_to(:assetable, :polymorphic => true)

 has_attached_file :asset,
                   :styles => {
                     :current => '1920x1080^',
                     :slide   => '@1920x400#',
                     :gallery => '800x600',
                     :thumb   => '400x370!',
                     :mini    => '55x55'
                   },
                   :convert_options => {
                     :mini => ' -quality 75 -strip' 
                   },
                   :hash_secret => "your-hash",
                   :default_url => "/assets/missing.gif",
                   :path => ":rails_root/public/system/:attachment/:id/:style/:digestive.:ext",
                    :url => "/system/:attachment/:id/:style/:digestive.:ext",
                    :processors => lambda { |i|
                      i.video? ? [:video_thumbnail] : [:thumbnail]
                    }

  def asset_get(type)
    asset.url(type)
  end

  def temp_thumbnail
    false
  end

Paperclip.interpolates(:digestive) do |attachment, style|
  attachment.instance.digested
end

  def digested
      "#{self.id}-#{digest_asset_name}"
  end


Paperclip.interpolates(:ext) do |attachment, s_name|
  case
    when ((style = attachment.styles[s_name]) && !style[:format].blank?) then style[:format]
    when attachment.instance.video? && s_name.to_s != 'original' then 'jpg'
  else
    File.extname(attachment.original_filename).gsub(/^\.+/, "")
  end
end

 def video?
    [ 'application/x-mp4',
    'video/mpeg',
    'video/quicktime',
    'video/x-la-asf',
    'video/x-ms-asf',
    'video/x-msvideo',
    'video/x-sgi-movie',
    'video/x-flv',
    'flv-application/octet-stream',
    'video/3gpp',
    'video/3gpp2',
    'video/3gpp-tt',
    'video/BMPEG',
    'video/BT656',
    'video/CelB',
    'video/DV',
    'video/H261',
    'video/H263',
    'video/H263-1998',
    'video/H263-2000',
    'video/H264',
    'video/JPEG',
    'video/MJ2',
    'video/MP1S',
    'video/MP2P',
    'video/MP2T',
    'video/mp4',
    'video/MP4V-ES',
    'video/MPV',
    'video/mpeg4',
    'video/mpeg4-generic',
    'video/nv',
    'video/parityfec',
    'video/pointer',
    'video/raw',
    'video/rtx' ].include?(self.asset.content_type)
  end

  private

  require 'digest/md5'
  def generate_name(name)
    Digest::MD5.hexdigest(name)
  end

  def digest_asset_name
    generate_name(self.asset_file_name)
  end

end

video_thumbnail.rb

# lib/paperclip_processors/

module Paperclip
  class VideoThumbnail < Processor

  attr_accessor :time_offset, :geometry, :whiny

  def initialize(file, options = {}, attachment = nil)
    super

    @current_file = file
    set_time_offset
    set_geometries
    set_whiny 
    set_basename 
  end

  def make
    create_tmp_file 
    exeception_rescuer { paperclip_runner(cmds) } 
    tmp_file
  end

  private

    attr_reader :basename, :tmp_file, :current_file

    def cmds
      cmd = %Q[-itsoffset #{time_offset} -i "#{file_expander(current_file)}" -y -vcodec mjpeg -vframes 1 -an -f rawvideo ]
      cmd << "-s #{geometry.to_s} " unless geometry.nil?
      cmd << %Q["#{file_expander(tmp_file)}"]
    end

    def file_expander(obj)
      File.expand_path(obj.path)
    end

    def paperclip_runner(cmd)
      Paperclip.run('avconv', cmd)
    end

    def create_tmp_file
      @tmp_file  = Tempfile.new([ basename, 'jpg' ].compact.join("."))
      tmp_file.binmode
    end

    def set_time_offset
      @time_offset = options[:time_offset] || '-4'
    end

    def set_whiny
      @whiny = options[:whiny].nil? ? true : options[:whiny]
    end

    def set_basename
      @basename = File.basename(file.path, File.extname(file.path))
    end

    def set_geometries
      @geometry = Geometry.parse(options[:geometry])
      assign_geometries unless geometry_options_nil? 
    end

    def assign_geometries
      geometry.width = (@geometry.width / 2.0).floor * 2.0
      geometry.height = (@geometry.height / 2.0).floor * 2.0
      geometry.modifier = ''
    end

    def geometry_options_nil?
      options[:geometry].nil?
    end

    def exeception_rescuer
      begin
        yield
      rescue Cocaine::CommandNotFoundError
        exeception_raiser if whiny
      end
    end

    def exeception_raiser
      raise PaperclipError, "There was an error processing the thumbnail for #{basename}"
    end

  end

end

Please checkout the last update.
I've added a new method to close the opened files.

:)