refactor: fix mimetype (planning#2251)

client/src/main/java/dev/fitko/fitconnect/api/domain/sender/SendableSubmission.java

@@ -138,7 +138,7 @@ public final class SendableSubmission {
 
         @Override
         public OptionalPropertiesStep setXmlData(byte[] data, URI schemaUri) {
-            this.dataMimeType = MimeType.APPLICATION_JSON;
+            this.dataMimeType = MimeType.APPLICATION_XML;
             this.schemaUri = schemaUri;
             this.data = getValidatedData(data);
             return this;