Shelf doesn't accept text files (TXT, MD, RTF) #85

Title: Shelf doesn't accept text files (TXT, MD, RTF)

Hi,

I think I found a bug in Shelf.

Environment:

• macOS Tahoe
• Latest SuperIsland version

Steps to reproduce:

1. Drag a TXT, MD, or RTF file into Shelf.
2. The file is not added to Shelf.

Expected:
Shelf should accept these files like any other file.

Actual:

• TXT ❌
• MD ❌
• RTF ❌
• DOCX ✅
• PDF ✅

According to SHELF.md:

"Files and folders are stored as metadata with security-scoped bookmarks where available."

So I would expect all file types to be accepted, including plain text documents.

Can you confirm whether this is a bug or intended behavior?

Thanks.