Check if the file already existe with another extension ?

[azerfr2]

In a groupchat, the file are in rar format and after download i decompress all of them, the result is the same filename with a different extension , could you help me with the syntax ?
original line is :
if _is_exist(file_name):

my attempt:
if _is_exist(file_name) or _is_exist(file_name.replace('rar', 'avi')) or _is_exist(file_name.replace('rar', 'mp4')):

2. is it correct the way i do to check for different extension in the same line , and or maybe there is a way to do in several line with some like in vba "case" statement like :

case .avi
case .mov
case .mp3