Convert endpoint may not take the correct mime type into consideration

[tagliala]

• The second parameter

  colore/lib/app.rb

  Line 188 in 1c743a3

  content = Converter.new(logger: @logger).convert_file(params[:action], body, params[:language])

  is not a file, but the file content, which is a string.
• If the string is plain text, the convert method might miss that it is actually text/markdown and not necessarily text/plain.
• The params[:file][:type] is indeed correct.

Reference: Issue #322