diff --git a/crates/idfoto-core/src/item_types/document.rs b/crates/idfoto-core/src/item_types/document.rs
index 5a16eb4..d6e8714 100644
--- a/crates/idfoto-core/src/item_types/document.rs
+++ b/crates/idfoto-core/src/item_types/document.rs
@@ -1,3 +1,40 @@
+//! Document: filename + mime + pointer to the primary attachment blob.
+
 use serde::{Deserialize, Serialize};
-#[derive(Debug, Clone, Serialize, Deserialize, Default)]
-pub struct DocumentCore {}
+
+use crate::ids::AttachmentId;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DocumentCore {
+    pub filename: String,
+    pub mime_type: String,
+    pub primary_attachment: AttachmentId,
+}
+
+impl Default for DocumentCore {
+    fn default() -> Self {
+        Self {
+            filename: String::new(),
+            mime_type: "application/octet-stream".into(),
+            primary_attachment: AttachmentId(String::new()),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn document_round_trip() {
+        let doc = DocumentCore {
+            filename: "passport.pdf".into(),
+            mime_type: "application/pdf".into(),
+            primary_attachment: AttachmentId("0123456789abcdef".into()),
+        };
+        let json = serde_json::to_string(&doc).unwrap();
+        let parsed: DocumentCore = serde_json::from_str(&json).unwrap();
+        assert_eq!(parsed.filename, "passport.pdf");
+        assert_eq!(parsed.primary_attachment.as_str(), "0123456789abcdef");
+    }
+}