Please solve the programming task below using a self-contained code snippet in a markdown code block.

Given an array of integers arr, find the sum of min(b), where b ranges over every (contiguous) subarray of arr. Since the answer may be large, return the answer modulo 109 + 7.
 
Please complete the following python code precisely:
```python
class Solution:
    def sumSubarrayMins(self, arr: List[int]) -> int:
```