Hello,
Please check the information available on https://www.phpdocx.com/en/forum/default/topic/2082. phpdocx uses PHP ZipArchive to generate DOCX documents (a DOCX is ZIP file). Sorry, but we don't know why DocuSign doesn't identify the DOCX documents correctly (an HTML is a plain file and a DOCX is a ZIP file), we recommend you to check the code you are using to send the document to DocuSign, maybe your code can send a mime-type (Content-Type) to set the document type so they can identify it as a DOCX document. On https://stackoverflow.com/questions/36569088/unable-to-upload-word-document-using-docusign-rest-api you can find a very similar question, although, with C#, that needs setting Content-Type and FileExtenstion to get DocuSign to work correctly.
Regards.