No, we don't want to make sure the customer selected the right file...

Actually, when the user upload more than one file at once, we need to to know how they are organized in the original file system, because then we apply some process to the uploaded files, which logic is based on their relative path between each others.

Since it is not possible to know the original path of the files uploaded, we must ensure that all the files uploaded at once are originately located in the same folder.