online fax customize cover sheet.

@Tyler Liu

I have a sandbox account .I am using Java SDK for sending Fax. Whenever i try providing my own custom cover page file in .docx. My Sending Fax Fails. If i remove coverPage by setting it to '0' it also fails.
Only Successfully sent when i go with Default Fax Page.

I am using below code to send Fax.

CreateFaxMessageRequest postParameters = new CreateFaxMessageRequest();
        MessageStoreCalleeInfoRequest[] recivingNumbers = new MessageStoreCalleeInfoRequest[updateMessage
                .getReceiverNumber().size()];
        for(int i=0;i<updateMessage.getReceiverNumber().size();i++) {
            recivingNumbers[i] = new MessageStoreCalleeInfoRequest()
                    .phoneNumber(updateMessage.getReceiverNumber().get(i));
        }
        
        postParameters.to = recivingNumbers;
        postParameters.faxResolution = "High";
        postParameters.coverPageText = StringUtils.isNotBlank(updateMessage.getContent()) ? updateMessage.getContent():"" ;
        
        if(!updateMessage.getFaxFileIds().isEmpty()) {
            List<Attachment> attachments = new ArrayList<>();
            if(Objects.nonNull(updateMessage.getFaxCoverPageId())) {
                FaxAttachment  coverPage = this.queueMessageService.getFaxFileAttachment(updateMessage.getFaxCoverPageId());
                postParameters.coverIndex = 0L; 
                Attachment coverPageObj = new Attachment(); 
                coverPageObj.bytes(coverPage.getData());
                coverPageObj.contentType(coverPage.getContentType());
                coverPageObj.fileName(coverPage.getFileName()); 
                attachments.add(coverPageObj);
            } 
            for(ObjectId id:updateMessage.getFaxFileIds()) {
                FaxAttachment  fileAttachment = this.queueMessageService.getFaxFileAttachment(id);
                Attachment attachmentPageObj = new Attachment(); 
                attachmentPageObj.bytes(fileAttachment.getData());
                attachmentPageObj.contentType(fileAttachment.getContentType());
                attachmentPageObj.fileName(fileAttachment.getFileName()); 
                attachments.add(attachmentPageObj);
            }
            
            Attachment[] attachmentsArr = attachments.stream().toArray(Attachment[]::new);
            postParameters.attachments = attachmentsArr;
        }
        
        FaxResponse response = restClient.restapi().account().extension().fax().post(postParameters);
        

Also is there any limitation on Number of Pages in fax for Sand Box Account? Currently fax with only 2 pages is getting successfully sent.
Image of my .docx file is as below