The correct MIME type for uploading XLSX files to the Vertex AI API is "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet".
Here is the code snippet you can refer to:

In the above code we are using the following key points:
- Uses the official MIME type for XLSX files: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet".
- Validates the uploaded file’s content type to ensure it’s an XLSX file.
- Uses Spring Boot’s MultipartFile to handle file uploads.
Hence, this approach ensures the correct handling and validation of XLSX files when uploading to the Vertex AI API, following the proper MIME type specification.