I struggle to find a way. Any advice is welcome. Thanks!
It’s not supported, AFAIK.
To be more specific, as per Understanding Uploads, Images, and Attachments - Site Management - Discourse Meta
The upload button’s icon changes depending on the allowed types of uploads. If only image file types are allowed, the upload icon will appear as an image icon.
However, if other file types are allowed, the upload icon will appear as a general upload icon.
Since it’s the former on here, you can only upload images.
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Interestingly, I cannot unmark the previous and mark this as Solution.
Right, I moved the post to meta, which doesn’t have a “solution” button 
all good.
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