How to Unassociate File Extension Types in Vista, Windows 7, and Windows 8
Information
This tutorial will show you how to unassociate a file extension to no longer have a default program to open it with afterwards in Vista, Windows 7, and Windows 8.
You must be logged in as an administrator to be able to unassociate file types.
Note
These are free standalone EXE program that run without installing anything on your computer.
OPTION ONE
Unassociate File Type using "FileTypesMan" Program
NOTE:This program supports running in XP, Vista, Windows 7, and Windows 8.
1. Download the ZIP file for the same 32-bit or 64-bit FileTypesMan at the bottom of the site below as your 32-bit or 64-bit Windows.
2. Open the downloaded filetypesman.zip or filetypesman-x64.zip, and extract (drag and drop) all files to the desktop.
3. Run the extracted FileTypesMan.exe file.
4. If prompted by UAC, click/tap on Yes (Windows 7/8) or Continue (Vista).
5. In the top pane, select one or more "extensions" (ex: .txt) that you wish to unassociate. (see screenshot below)
Note
To select random multiple file extensions:
Press and hold the Ctrl key while clicking on extensions to select them, then release the Ctrl key when finished.
To Select everything inbetween two selected file extensions including the selected file extension:
Select an extension, press and hold the Shift key, click on another extension to select them and everything inbetween them, then release the Shift key.
6. When finished with step 5, click/tap on Edit (menu bar) and Delete Selected Extensions. (see screenshot below)
7. Click/tap on Yes to confirm. (see screenshot below)
8. When finished, you can close FileTypesMan if you like.
OPTION TWO
Unassociate File Type using "Unassociate File Types" Program
NOTE:This program only supports running in Windows 7 and Vista. Windows 8 is not supported.
1. Download the ZIP file for the free Unassociate File Types program under Attachements towards the bottom of the site at the link, and save it to your desktop.
2. Open the downloaded unassoc.zip, and extract (drag and drop) all files to the desktop. (see screenshot below)
3. Run the extracted Unassoc.exe file.
4. If prompted by UAC, click/tap on Yes (Windows 7) or Continue (Vista).
5. To Unassociate a File Extension Type
A) Click/tap on a file extension type from the list in the left pane to select it. (see screenshot below step 7)
B) Click/tap on Remove file association (User). (see screenshot below step 7)
6. To Delete a File Extension Type
A) Click/tap on a file extension type from the list in the left pane to select it. (see screenshot below step 7)
B) Click/tap on Delete file type. (see screenshot below step 7)
Do you have a .dm file extension listed instead? That would be the actual file extension instead of .sis.dm. Only the letters after the the last dot is the file extension.
Do you have a .dm file extension listed instead? That would be the actual file extension instead of .sis.dm. Only the letters after the the last dot is the file extension.
uh i got it
there is a .dm extension listed, but when i selected the remove file associations (user) button is inactive :S
I attached a REG file below that will remove the .dm file extension from the usual locations in the registry that should also unassociate it. After merging the REG file, restart the computer to fully apply.
Information
Note
OPTION ONE 
Quote: Originally Posted by Brink 
Unassociate a file type 
