When you try to download an unknown type of file from the Web or in an email attachment, your computer may identify it as an "octet-stream" file. Octet-stream does not refer to a specific type of file -- it could be anything from a spreadsheet to an executable program. To open an unidentified file, you need to either figure out which program can open the file as a document or change the file's extension to run as a program.
Files With Extensions
Double-click the octet-stream file after downloading it. If the file contains a usable extension, it may load correctly even if your email client or Web browser couldn't identify it. Otherwise, Windows will display a list of programs to try using.
Click a program in the list to load the file using the selected software. If you don't see any options in the list other than "Look for an app in the Store," click "More Options." To select a specific program not found on either list, click "Look for Another App on This PC."
Check whether the file loads correctly. If it doesn't, quit the program and try again with another application.
Double-click the file again, once you find a working application. Check the box to "Use this app" for all similar files and pick the correct program. This associates the extension with the selected application for future use.
Files Without Extensions
Double-click the file to display a list of programs on your computer. Pick one to try loading the file. If the file does not load correctly, quit the program and try another.
Select the file after figuring out its type. Press "F2" to rename the file and type its extension after the file name. For example, if you discover a file to be a Word 2013 document, add ".docx" without quotes, then press "Enter." The typed extension will disappear but the file's icon will change, identifying its new type.
Append the extension ".exe" if you need to run a file as an application, rather than as a document in another program. Files that appear as octet-stream files are often applications, but you should only run a downloaded application if you trust its source.
The only way to know what program can open an unfamiliar file type is through prior knowledge -- such as a description by the person who sent you the file -- or by researching the file's extension. Keep in mind that you may not have any programs that can load the file.
If you add the wrong extension to a file, you can't fix it by typing a new extension after the file name. Instead, hold "Shift," right-click the window containing the file and pick "Open Command Window Here." At the prompt, type "rename "[filename].[old-extension]" "[filename].[new-extension]"" without brackets or external quotes and press "Enter."
If you accidentally select the wrong program to always open a file type, right-click the file, choose "Open With" and then click "Choose Default Program."
Information in this article applies to Windows 8 and 8.1. It may vary slightly or significantly with other versions or operating systems.