Allow storing arbitrary files with the recipe

Is your feature request related to a problem? Please describe.

Someone who downloads recipes in PDF format would like to store these PDFs with the recipe so they don't get lost. Of course one could just copy and paste the text, but since the PDFs are designed with love it would be a pity to discard them.

Describe the solution you'd like

A simple solution like this could work:

1. #443 is done so multiple images can be uploaded.
2. The image upload dialogue allows arbitrary files instead of just images.
3. Any attachment that doesn't seem to be an image might be stored as a MediaObject and is shown below the image(s) in a line like this:

Attachments: file1.pdf, file2.doc

Describe alternatives you've considered

• Discard the designed PDF files.
• Store them somewhere else and mention it in the instructions.
• Save the PDF as PNG and attach it (not ideal for printing).

Related to #507 and to #340.

This would also help with the inevitable issue of failed imports from a URL. No matter how good the scraper becomes, there will always be SOME pages that can't be parsed, for any number of reasons. The alternative in those cases is to enter the whole recipe by hand, but that's not always feasible in the moment.

The next most optimal action for me would be to use a site's Print function to produce a PDF and attach it to a Nextcloud Cookbook recipe for safekeeping. Maybe the original URL will become importable later, or maybe I'll have the time to enter it by hand, or maybe I'll never get around to it, but at least I HAVE it in some form.

Additionally, this could accommodate, however clumsily, all kinds of other sources: Handwritten recipes, scans of books, photos someone took of post-its on their fridge; we'd be able to create rudimentary entries for all of them.