In this age of cloud documents, we often send links to documents instead of attaching files.
My suggestion is that an attached file could be a URL or an uploaded file, depending on the user’s preference.
If possible, Vikunja could try and ascertain the title of the URL target (e.g. if a web page), or the user could specify a title for the attached URL.
The user might even select one of a small number of icons for the attachment (e.g. document, spreadsheet, web page, video, etc.).