All file extensions which open with the same program may switch to the same icon. If that happens, this is one of the work-arounds:

1. Create a folder and inside it create an empty .exe file for each individual file extension. For example, create folder C:\Windows\IconSet\
and inside it create an FLV.exe empty file.

2. Right click on an any *.flv file > Open with > Choose default program... > Select a program from a list of installed programs > OK > Browse to the empty FLV.exe file > OK 
(Make sure you CHECK: Always use the selected program to open this kind of file)

3. Start Types > Double click on .flv > Actions TAB > Click on Open then click on the yellow folder and browse to the program you actually want to use to open .flv files > Then go to Types > Icon TAB to set the custom icon.