the same way you add PDF files: just drag and drop to Zotero or use "Store copy of file" or "Add link to file" depending on which you want (both are under the green plus sign).
I think maybe you're looking in the wrong place somehow? If you sort your library by Date Added (for standalone attachments) or expand the parent items (for child attachments), you don't see the files?
If you really think they're not there, can you provide a Debug ID for an attempt to add a file that doesn't work?
Also, if you have Firefox installed, have you checked whether this happens in Zotero for Firefox? (It will share your existing data directory by default, so it's easy to test. You just need to close Standalone first.)
Upgrade Storage
I cannot add any files except for pdf (for example, epubs) into my standalone zotero.
I reinstalled zotero and restarted my computer, it did not help.
Steps to reproduce:
1. Open zotero standalone
2. Drag a epub file into it.
OR
3. Click on the green plus
4. Chose "Book"
5. Right click on the item that appeared
6. "Add attachment" -> tried all three options, nothing works
By nothing works I mean there is no effect, the file does not attach.
If you really think they're not there, can you provide a Debug ID for an attempt to add a file that doesn't work?
The Debug ID is D1003926017