Change file type

Does anyone knows how to change the setting of the windows so that i can change the file type of a file by changing it's name.. like for example.. a file's name is sdlfsd.avi.bc! how can i change the setting of windows so that i can view the file type wif it's name coz now i will view the file as sdlfsd.avi without the format and i wan to change the format by juz renaming it if anyone noe wat i mean.. can u plz help/? Thanz alot in advance and i'm using window xp.. Thanz..

Uncheck "hide extentions for known file types" in folder options > view

is that a bitcomet file? if it is with that extension then it probably isnt finished downloading or its corrupted. u probably wont b able to open it if im right with any media player except maybe something like avi preview or i dont remember if bitcomet has a media player in it to let u preview imcompletes and just renaming to avi i doubt will work either unless its finished and just didnt get renamed for some reason

Yeah.. it's a bitcomet file and i change it.. i can be view but only up to where i downloaded.. Thanz alot for both ur replys.. great helps.