Mathematica file name problem

I have been having a problem lately with a program called Mathematica made by Wolfram Research. The files showed up in the my documents as unkown type, but the real file name is .NB. Everytime i try to open it up from the my documents folder as a .nb file it does not work. and now, i mistakenly changed it to an internet exlplorer file. How do I change it back to a .NB file???? I can only open my files up by starting Mathematica first.

 

first off you might wanna throw this kinda question in the software forum next time. and as for your prob, before anybody gets technical, have you tryed going to save as, and changing the file type at the bottom? if so what types of files are available for it to be converted to? this is not sarcasim, its just smart to get the simple stuff out of the way first.