fixed problem with mime being null in Attachment

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

de.srsoftware.cal.api/src/main/java/de/srsoftware/cal/api/Attachment.java

@@ -12,6 +12,6 @@ import org.json.JSONObject;
  */
 public record Attachment(URL url, String mime) {
 	public JSONObject json() {
-		return new JSONObject(Map.of("url", url.toString(), "mime", mime));
+		return new JSONObject(Map.of("url", url.toString(), "mime", mime == null ? "content/unknown" : mime));
 	}
 }

de.srsoftware.cal.importer/src/main/java/de/srsoftware/cal/importer/weimar/CKeller.java

@@ -0,0 +1,4 @@
+package de.srsoftware.cal.importer.weimar;
+
+public class CKeller {
+}