Cannot handle XLS/XSLX files using API Gateway requests

First of all, I am very new to **AWS Lambda/Serverless** and **AWS API GW** so I am trying to develop a simple handler func which can be invoked using POSTMAN and can POST a XLS file on my local using **AWS SAM**

My handler is of the below signature:
```
func Handler(request events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {
```
It can decipher a CSV file but can't fro XLS file for which the `request.Body` is null.

May I know what's the problem?