ASSOC - Change file extension associations - Windows CMD Commands

Display or change the association between a file extension and a fileType
Syntax   ASSOC .ext = [fileType]   ASSOC   ASSOC .ext   ASSOC .ext =
Key    .ext      : The file extension    fileType  : The type of file A file extension is the last few characters in a FileName after the period. So a file called JANUARY.HTML has the file extension .HTML
The File extension is used by Windows NT to determine the type of information stored in the file and therefore which application(s) will be able to display the information in the file. File extensions are not case sensitive and are not limited to 3 characters.
More than one file extension may be associated with the same File Type.e.g. both the extension .JPG and the extension .JPEG may be associated with the File Type "jpegfile"
At any one time a given file extension may only be associated with one File Type.e.g. If you change the extension .JPG so it is associated with the File Type "txtfile" then it's normal association with "jpegfile" will disappear. Removing the association to "txtfile" does not restore the association to "jpegfile"
File Types can be displayed in the Windows Explorer GUI: [View, Options, File Types] however the spelling is usually different to that expected by the ASSOC command e.g. the File Type "txtfile" is displayed in the GUI as "Text Document"and "jpegfile" is displayed as "image/jpeg"
The command ASSOC followed by just a file extension will display the current File Type for that extension. 
ASSOC without any parameters will display all the current file associations.
ASSOC with ".ext=" will delete the association for that file extension.
Did you leave the Always Use This Program To Open This File option turned on? To change it back so it prompts you to specify a program each time, just delete the association for that file typeASSOC .ext=[where .ext is the file extension].Now when you double-click on a file of that type, the system will ask you what program you want to use. 
Using the ASSOC command will edit values stored in the registry at HKey_Classes_Root\.<file extension> Therefore it's possible to use registry permissions to protect a file extension and prevent any file association changes.
Examples:
Viewing file associations:
ASSOC .txtASSOC .docASSOC >backup.txt
Editing file associations:
ASSOC .txt=txtfileASSOC .DIC=txtfileASSOC .html=Htmlfile
Deleting a file association: 
ASSOC .html=
Repair .REG and .EXE file associations:
ASSOC .EXE=exefileASSOC .REG=regfile
Digging through CLASSES_ROOT entries often reveals more than one shell for the same application, for example the Apple Quick Time player has two entries, one to "open" (which gives an annoying nag screen) and one to just "play" the QT file:[HKEY_CLASSES_ROOT\MOVFile\shell\open] and [play]
In cases like this you can change the default action e.g.[HKEY_CLASSES_ROOT\MOVFile\shell]@="play"