Hey Carolina,
That is where the SessionId property comes in. You can set it to something like "Post" + PostId (assuming PostId is a variable on your screen) and then put a filter on the UploadCache aggregate that uses the SessionId.
Also, something to be aware of, the UploadCache is only a temporary storage location. There is a timer in the MultipleFileUpload component that will clear files that are older than 20 minutes. So, in your On Notify, you can use an aggregate with a filter against SessionId and then save the file to somewhere more permanent (e.g. your own table, Amazon S3, Azure Storage, etc.)