Attaching files with non-ASCII file name is broken

When editing a page and attaching files using the Files menu button, files that contain non-ASCII characters (e.g. Swedish umlauts like åäö) break when added to the page, being replaced with "Unknown attachment" on the page. The attachment itself is still in the list.

Drag-n-drop or adding through Browse inside Files both break the attachment. Drag-n-drop directly to the page breaks too, except with an image file (PNG), which seems fine to add directly to the page but not through Files. Oddly, drag-n-drop to the page seems to work at first for the other file types too, until you press Save on the page.

Long file names or spaces seem OK. DOCX, PDFD, TXT and PNG file types tested.

Se screenshots for testing matrix on a live page.