Uploads a PDF or Word file (.pdf, .docx) of at most 50 MB to a draft envelope as multipart/form-data and returns the new content item. Larger files are rejected with 413. The file type is taken from the content type of the fileData part, so send application/pdf or the .docx MIME type. Files can only be added while the envelope status is DRAFT. To fill a Word template, send templateDataJsonString. To send base64 files in a JSON body instead, use Create and Publish Envelope.