POSTGRESQL Insert Multiple Rows from given JSON array, use results from insert to do other inserts

TL;DR Music Streaming app - allows users to upload their own files. Files will be stored in a FileSet. I want to pass in more than one FILE(array of fileRecords), with one FILESET, and insert into the appropriate tables.

Sorry if this is a little confusing - basically I am working with 3 tables:

File table

CREATE TABLE file
(
    -- Most tables should have a table_id & table_cuid
    file_id         BIGSERIAL   NOT NULL ,
    file_cuid       VARCHAR     NOT NULL,

    -- This section is specific to this table
    user_id         BIGINT      NOT NULL,

    -- File fields
    filename        VARCHAR     NOT NULL,
    last_modified   TIMESTAMP WITHOUT TIME ZONE NOT NULL,
    size_in_bytes   VARCHAR     NOT NULL,
    mime_type       VARCHAR     NOT NULL,

    -- Most tables should also have timestamps
    created_timestamp  TIMESTAMP WITHOUT TIME ZONE  NOT NULL,
    updated_timestamp  TIMESTAMP WITHOUT TIME ZONE  NOT NULL,

    -- Constraints
    PRIMARY KEY (file_id),
    FOREIGN KEY (user_id)   REFERENCES users (user_id),
    CONSTRAINT file_cuid_unique  UNIQUE     (file_cuid)
)

FileSet Table

CREATE TABLE fileset
(
    -- Most tables should have a table_id & table_cuid
    fileset_id         BIGSERIAL   NOT NULL ,
    fileset_cuid       VARCHAR     NOT NULL,

    user_id         BIGINT      NOT NULL,

    -- Fileset fields TBD

    created_timestamp  TIMESTAMP WITHOUT TIME ZONE  NOT NULL,
    updated_timestamp  TIMESTAMP WITHOUT TIME ZONE  NOT NULL,

    -- Constraints
    PRIMARY KEY (fileset_id),
    FOREIGN KEY (user_id)   REFERENCES users (user_id),
    CONSTRAINT cuid_unique_fileset  UNIQUE     (fileset_cuid)
)

and finally a File_FileSet_Rel Table

CREATE TABLE file_fileset_rel
(
    file_id         BIGSERIAL   NOT NULL,
    fileset_id      BIGSERIAL   NOT NULL,

    FOREIGN KEY (file_id)  REFERENCES file   (file_id),
    FOREIGN KEY (fileset_id)   REFERENCES fileset (fileset_id),
    UNIQUE      (file_id, fileset_id)
)

Every song will have one FileSet associated with it, and that fileset has one/multiple files associated.

The function below (file_upload_process) allows me to add the File to the table (returning file_id), as well as adding the Fileset to the table (returning fileset_id), and finally the function inserts both of these values into the final table file_fileset_rel

file_upload_process.sql

CREATE OR REPLACE FUNCTION file_upload_process(
  _file_cuid VARCHAR,
  _fileset_cuid VARCHAR,
  _cognito_id VARCHAR,
  _name VARCHAR,
  _last_modified TIMESTAMP,
  _size DECIMAL,
  _type VARCHAR
)
RETURNS TABLE(
  fileId BIGINT,
  fileSetID BIGINT
)
AS $file_upload_process$
DECLARE
v_user_id BIGINT;
v_file_id BIGINT;
v_fileset_id BIGINT;
BEGIN

-- Create Song/ Update song
  -- store user_id
  v_user_id := (
    SELECT * FROM get_user_id(_cognito_id)
  );

  -- --Insert In FILE table
  v_file_id := (
    SELECT *
    FROM create_file(
    _file_cuid,
    v_user_id,
    _name,
    _last_modified,
    _size,
    _type
    )
  );

  --Insert into fileset table
  v_fileset_id := (
  SELECT * FROM create_fileset(_fileset_cuid, v_user_id)
  );

  RETURN QUERY
  INSERT INTO file_fileset_rel(file_id, fileset_id)
  VALUES (
    v_file_id,
    v_fileset_id
  )
  RETURNING file_fileset_rel.file_id,
            file_fileset_rel.fileset_id;
END;
$file_upload_process$ LANGUAGE PLPGSQL;

Which calls the following functions within:

create_file.sql

CREATE OR REPLACE FUNCTION create_file(
  _file_cuid VARCHAR,
  _user_id BIGINT,
  _name VARCHAR,
  _last_modified TIMESTAMP,
  _size DECIMAL,
  _type VARCHAR
)
RETURNS TABLE (
  _file_id BIGINT
)
AS $create_file$
BEGIN
  RETURN QUERY
-- Create Song/ Update song
  INSERT INTO file(
    file_cuid,
    user_id,
    filename,
    last_modified,
    size_in_bytes,
    mime_type,
    created_timestamp,
    updated_timestamp
  )
  VALUES(
    _file_cuid,
    _user_id,
    _name,
    _last_modified,
    _size,
    _type,
    now(),
    now()
  )
  RETURNING "file".file_id;
END;
$create_file$ LANGUAGE PLPGSQL;

And the create_fileset function:

    CREATE OR REPLACE FUNCTION create_fileset(
  _fileset_cuid VARCHAR,
  _user_id BIGINT
)
RETURNS TABLE (
  _fileset_id BIGINT
)
AS $create_fileset$
BEGIN
-- Create Song/ Update song
  RETURN QUERY
  INSERT INTO fileset(
    fileset_cuid,
    user_id,
    created_timestamp,
    updated_timestamp
  )
  VALUES(
    _fileset_cuid,
    _user_id,
    now(),
    now()
  )
  RETURNING "fileset".fileset_id;
END;
$create_fileset$ LANGUAGE PLPGSQL;

The issue I'm having is that this function only allows you to add 1 file at a time, users could upload 5-10 files for 1 song (1 fileset) and so I need a way to loop through the array of fileRecords and store the values of those fields in the FILE table columns - finally allowing me to add multiple records to the file_fileset_rel table.

If anyone has easier ways to do all of this, or more importantly a solution to the MANY vs One insert would be a huge help.

EDIT: Example of CURRENT body being sent to lambda - before passing to postgresql function URL -> fileset/{filesetCuid}/file

body: {
"cognitoId" : "9dc766a0-c5c9-4455-932e-46c243c80266",
"fileRecord" : {
    "fileCuid" : "someotherfilecuid23",
    "name" : "file_name",
    "last_modified" : "06/03/1995",
    "size" : 530,
    "type" : "mp3/audio"
}
}

End goal - send multiple files: URL -> fileset/{filesetCuid}/file

body: {
"cognitoId" : "9dc766a0-c5c9-4455-932e-46c243c80266",
"fileRecords" : [{
    "fileCuid" : "someotherfilecuid23",
    "name" : "file_name",
    "last_modified" : "06/03/1995",
    "size" : 530,
    "type" : "mp3/audio"
}, {...}, {...}]
}