SharePoint Online Rest API call returning Empty

I am trying to get all the files in a document library placed in a subsite. The document library is called WorkInstructions. Using Bearer access token.

Below is the API call using :

https://tenant.sharepoint.com/teams/manu/mfgoper/_api/web/GetFolderByServerRelativeUrl(%27/teams/manu/mfgoper/WorkInstructions%27)/Files

I can get the item count just not the data. Any help is appreciated.