Feature request: upload file

[giplt]

Add a new datatype: file E.g. pdf, datasets, ... discussion needed on which filetypes we consider to be 'safe' to host

Use cases:

• attach documents to an article or web page
• upload files to a resource bin for investigative journalism project

[tammoterhark]

Another use case: add file to mailing for mailing list

[matthijskooijman]

Related: #122 about uploading audio and video (which is different from plain files in that they should probably be able to play from the browser).