Allow storing arbitrary files with the recipe

[rompe]

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. Enable to get the possibility to save more than one picture #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).

[christianlupus]

Related to #507 and to #340.