FileId for GRAB API:

FileId in the grab payload is the parent folder path of the template file. For example, suppose you have a nested folder called Folder1/Folder2 that contains a file called test1.docx in it. The FileId looks like this: Folder1/Folder2/test1.docx

FileId for SEND API:

FileId in the Send payload is the parent folder path. For example, suppose you have a nested folder called Folder1/Folder2 where you want to send a template file. The FileId looks like this: Folder1/Folder2