Upload of xml.xz file denied as subitem

[sebix]

1. Go to any page
2. Go to the subitems
3. Click New Item
4. Click Browse
5. Select an xz-compressed XML file
6. The response is: UnicodeDecodeError, upload failed, not a text file, nothing saved: 'short.xml.xz'. Try changing the name.

The wiki allows uploading binary files, so an xz file should be allowed. And xz is clearly not a text file, so the error message is wrong or misleading.

Uploading the file as short-xml.xz works, so there may be XML parsing triggers where they shouldn't be.