FireCloud uses tab-separated-value (TSV) files to import sample, participant, pair, and sets of these entities and attributes into the Data tab. Each line in the TSV file corresponds to a sample, participant, pair, or set entity and must reference entities of the same type. This means one file per type that you are trying to upload. To read more about the format required to upload these entities check out the linked articles:
Order for uploading Load Files
Load files must be imported in a strict order due to references to other entities.
The order is as follows ("A > B" means entity type A must precede B in order of upload):