count files extensions files for each month based on modified date

[June7]

Your code needs to add a new row for each filetype/month pair? This means the dictionary must add a new item for each ext/month. You are only testing if ext exists, need to test for the pair. And if you want multi-year output, that means a third criteria to group by.

What is purpose of ltst and fin date variables?

Use InStrRev() instead of InStr() to locate last period in file name since there could be multiple periods (bt_21.10.1_64_win10.exe).
InStrRev(Filename, ".") + 1

Also don't see iteration through subfolders. Review http://allenbrowne.com/ser-59.html

This procedure needs significant re-working.