1) Open notepad
2) Type this in -> "C:\Program Files (x86)\Warcraft III\Warcraft III" -loadfile %1
3) File -> Save as
4) Save as file type -> All files
5) File name -> WatchReplay.bat
6) Save to war3 folder
7) Right click the replay you want to watch -> Open with
8) Click -> choose default location
9) Click -> Browse
10) Go to war3 folder
11) Open WatchReplay.bat
12) Tick checkbox -> Always use the selected program to poen this kind of file
Instructions are for windows7, but win10 shouldn't be much different.
Spoiler!
-Burn