In the POST man tool select the binary option where it will allow to attach or browse any file - there you attach any excel file by browsing your local directory - then perform the POST operation to SharePoint, this will upload the document to the SharePoint library - then go to the code section of the POST man tool where we can all equivalent code for this POST operation in almost all coding language like C#, JSON, REST API etc. Then look at the C# coding structure how it got generated automatically and we can use this format of code wherever we want.
Binary option selection example:

How to get the code snippet from POST man tool:

Example:
Using the below C# REST API code we can add file to SharePoint:
var fileName = Path.GetFileName(uploadFilePath);
var requestUrl = string.Format("{0}/_api/web/Lists/GetByTitle('{1}')/items({2})/AttachmentFiles/add(FileName='{3}')", webUrl, listTitle, itemId,fileName);
var request = (HttpWebRequest)WebRequest.Create(requestUrl);
request.Credentials = credentials; //SharePointOnlineCredentials object
request.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
request.Method = "POST";
request.Headers.Add("X-RequestDigest", requestDigest);
var fileContent = System.IO.File.ReadAllBytes(uploadFilePath);
request.ContentLength = fileContent.Length;
using (var requestStream = request.GetRequestStream())
{
requestStream.Write(fileContent, 0, fileContent.Length);
}
var response = (HttpWebResponse)request.GetResponse();
Reference URL:
upload attachment to SharePoint List Item using REST API from Java client
https://social.msdn.microsoft.com/Forums/office/en-US/40b0cb04-1fbb-4639-96f3-a95fe3bdbd78/upload-files-using-rest-api-in-sharepoint-2013?forum=sharepointdevelopment