Having problems to make this work with Rail 6.0.3.2

[jan-gerritsen]

All was working fine in Rails 4, but I'm currently upgrading to Rails 6.0.3.2 (with Ruby 2.6.6), and riif v1.0.0 and now the line

    respond_to do |format|
      format.iif { render iif: render_to_string, filename: 'myfile' }
    end

gives me this error:
ArgumentError (Invalid formats: #<Mime::Type:0x00007fc16304ceb0 @synonyms=["application/qbookspro", "text/iif"], @symbol=:iif, @string="application/qbooks", @hash=4387365843056142794>):

So it's not that the MIME type did not get registered (otherwise Rails would not know about the application/qbookspro, but something else. I'm very much confused.

[linjunpop]

Would you please set up an example app to reproduce this issue? Thanks.