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There is list of programs that support DO file extension http://www.howopen.org/en/extensions/web-files/do
.XLS files are standard Microsoft Excel spreadsheet files. .XLT extension is used for the Excel template files. - Neeraj Sharma
Picture files no matter name they have they will definitely have an extension like xx.jpg . bmp and may more. window automatically will search for these files extension if asked to do so.
.b is an extension for binary files mostly used by video game emulators... .b is also the extension for BASIC programming language source text files...
.exe
msc
.fla
A compiled Help File in Windows will have the extension .chp
An applet is a Java application, this saves files in a .class extension
JAR files are simply ZIP files with a different extension. Just chnage the extension from .zip to .jar.
Those files have the extension .java