«أَمَدَّكُمْ بِمَا تَعْلَمُونَ»
He aided you with blessings known to you.
«أَمَدَّكُمْ بِأَنْعَامٍ وَبَنِينَ»
He aided you with livestock and sons.
«وَجَنَّاتٍ وَعُيُونٍ»
And with gardens and springs.
The phrase «بِمَا تَعْلَمُونَ» initially refers to the blessings in general terms. The next sentence identifies noteworthy members of that general class. It thus functions as an explanatory substitute; faṣl lets the detailed statement stand as a more focused expression of the original intention.