Eliminate intermediate columns

Here is one way
enter avi in C2 and mp4 in C3
then use formula

Formula:

=COUNTIF(B:B,"*"&C2)

v	B	C	D
1
2	Dark-Shadows-0247.avi	avi	14
3	Dark-Shadows-0248.mp4	mp4	4
4	Dark-Shadows-0249.mp4
5	Dark-Shadows-0250.mp4
6	Dark-Shadows-0251.avi
7	Dark-Shadows-0252.avi
8	Dark-Shadows-0253.avi
9	Dark-Shadows-0254.avi
10	Dark-Shadows-0255.avi
11	Dark-Shadows-0256.avi
12	Dark-Shadows-0257.avi
13	Dark-Shadows-0258.avi
14	Dark-Shadows-0259.avi
15	Dark-Shadows-0260k.avi
16	Dark-Shadows-0261.avi
17	Dark-Shadows-0262.avi
18	Dark-Shadows-0263.mp4
19	Dark-Shadows-0264.avi