How to Handle Files With No Extension
What about files with no extension?
Unlike the Macintosh which embeds creator information into files so they can have just about any name, a PC still mostly uses file extensions to associate programs with files. But, what do you do with a file that has no extension? The simple answer is: punt.
With no extension there can be no direct association. So, you have to know exactly what the file's format is. You can only know this by either looking into the file and trying to figure it out or, easier, sending a note to the person who sent you the file asking what program created it and what format it's in. With that information you can determine if you can open the file or ask to have it re-sent in a format you can open.
One other alternative would be to run the program TrId against the file(s). TrId is a program that attempts to determine a file's type based on known formats. (There is also an online version of TrID.)
Even given that you find the appropriate information, you still can't directly associate a single file with no extension to a given program and if you could in general, you probably would not want to as there are many different types of files that don't have an extension. Open the proper program and then use the File | Open command in the program's menu to open the file(s) you need to open. Also, search the program to see if it has a default file extension and if so, rename the file in question with that file extension.
[Note: If you can't rename the file with a different extension make certain that common file extensions are set to show. See the FAQ here for instructions.]