I copied the use of ';' as a file list delimiter where it is used exactly the same way if multiple files are selected for download, delete, etc.

Therefore I thought that this might be a safe way to do it. But I just saw that we also allow filenames with ';' which could lead to problems. I will switch to '|' which is not allowed in file names. Probably we should fix this in the files app too. But that's another topic, maybe something for @icewind1991...