Custom validation for file size in rails before being uploaded

in my form i have

<%= label_tag("file", "Attachment:") %><%= file_field_tag "uploadfile" %>

In my model i would like to write this

validate :validates_uploadfile

def validates_uploadfile(file)
    max_size = 2048
    errors.add(:uploadfile, "File size exceeds limitation") if file.size > max_size
end

In my controller can i call something like this

validates_upload_file(params[:uploadfile])

Is there a way to validate a file upload before being uploaded(not by using javascript or by looking at the file extension)
Thanks for the help

UPD

validate :uploadfile_validation, :if => "uploadfile?"

def uploadfile_validation
    errors[:uploadfile] << "should be less than 1MB" if uploadfile.size > 1.megabytes
end