simple fix

hapi-fhir-base/src/main/java/ca/uhn/fhir/util/AttachmentUtil.java

@@ -24,6 +24,7 @@
 import ca.uhn.fhir.context.BaseRuntimeElementDefinition;
 import ca.uhn.fhir.context.FhirContext;
 import ca.uhn.fhir.context.FhirVersionEnum;
+import ca.uhn.fhir.model.primitive.CodeDt;
 import org.apache.commons.lang3.Validate;
 import org.hl7.fhir.instance.model.api.IBase;
 import org.hl7.fhir.instance.model.api.ICompositeType;
@@ -41,8 +42,8 @@ public static IPrimitiveType<byte[]> getOrCreateData(FhirContext theContext, ICo
 		return getOrCreateChild(theContext, theAttachment, "data", "base64Binary");
 	}
 
-	public static IPrimitiveType<String> getOrCreateContentType(FhirContext theContext, ICompositeType theAttachment) {
-		return getOrCreateChild(theContext, theAttachment, "contentType", "string");
+	public static IPrimitiveType<CodeDt> getOrCreateContentType(FhirContext theContext, ICompositeType theAttachment) {
+		return getOrCreateChild(theContext, theAttachment, "contentType", "code");
 	}
 
 	public static IPrimitiveType<String> getOrCreateUrl(FhirContext theContext, ICompositeType theAttachment) {

hapi-fhir-docs/src/main/resources/ca/uhn/hapi/fhir/changelog/7_0_0/5537-attachment-util-content-type-fix.yaml

@@ -0,0 +1,5 @@
+---
+type: fix
+issue: 5537
+title: "Calling the method getOrCreateContentType in AttachmentUtil on an attachment with no content type would throw exception because contentType is a code not a string. 
+This fixes the function to create an empty code as expected"

hapi-fhir-jpaserver-test-dstu3/src/test/java/ca/uhn/fhir/validator/AttachmentUtilTest.java

@@ -1,11 +1,16 @@
 package ca.uhn.fhir.validator;
 
 import ca.uhn.fhir.context.FhirContext;
+import ca.uhn.fhir.model.primitive.CodeDt;
 import ca.uhn.fhir.util.AttachmentUtil;
 import org.hl7.fhir.instance.model.api.ICompositeType;
+import org.hl7.fhir.instance.model.api.IPrimitiveType;
+import org.hl7.fhir.r4.model.Attachment;
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 
 public class AttachmentUtilTest {
 
@@ -53,4 +58,18 @@ public void testCreateAttachmentR5() {
 		String encoded = ctx.newJsonParser().encodeResourceToString(communication);
 		assertEquals("{\"resourceType\":\"Communication\",\"payload\":[{\"contentAttachment\":{\"contentType\":\"text/plain\",\"data\":\"AAECAw==\",\"url\":\"http://foo\",\"size\":123}}]}", encoded);
 	}
+
+	@Test
+	public void testGetOrCreateContentTypeOnEmptyAttachmentR4(){
+		FhirContext ctx = FhirContext.forR4Cached();
+		Attachment attachment = (Attachment) AttachmentUtil.newInstance(ctx);
+
+		assertNull(attachment.getContentType());
+
+		IPrimitiveType<CodeDt> contentType = AttachmentUtil.getOrCreateContentType(ctx, attachment);
+
+		contentType.setValueAsString("text/plain");
+
+		assertNotNull(attachment.getContentType());
+	}
 }