ActionDispatch::Http:Parameters force encoding of all params but filename of an uploaded file

I run into a problem when I submit a form with a text field and a filename of an uploaded file both containing special chars.

Tracking down the issue it seems that while ActionDispatch force the encoding of all string paramaters to UTF-8 (see ActionDispatch::Http:Parameters encode_params private method), it does not do at to the filename of the uploaded file.

params is then filled with two different encoding (UTF-8 and ASCII-8BIT) causing an Encoding::CompatibilityError later on Arel-2.0.8 (to_sql.rb visit_Arel_Nodes_Values method).

I monkey patched ActionDispatch::Http:UploadedFile like so and it works :

  def initialize(hash)
    @original_filename = hash[:filename].force_encoding("UTF-8")        
    @content_type      = hash[:type]
    @headers           = hash[:head]
    @tempfile          = hash[:tempfile]
    raise(ArgumentError, ':tempfile is required') unless @tempfile
  end

My computing skill stop here. I let you guys decide if this is a bug or if I'm doing something wrong.