When \Control Panel\All Control Panel Items\Default Programs\Set Associations\ does not know which app should open a file type, it assigns a blank-sheet-of-paper icon to the filename. In my case, I use a popular .html editor called NoteTab (Fooke's Software Co.), and one of its internal Dictionary file types is .udt. When I inspected the NoteTab file C:\Program Files (x86)\NoteTab 7\Dictionaries\Default.udt, I found the blank-sheet-of-paper icon, clicked on the context menu itemOpen with, and forgot to uncheck the box Always use the selected program to open this kind of file, fromChoose the program you want to to use to open this file:. So, I ended up binding .udt to NoteTab 7 in \Default Programs\Set Associations\. This might not otherwise be a big potential problem, were it not for a registry search on ".udt" yielding the following 9 keys ... HKEY_CURRENT_USER\Software\Classes\.udt HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.udt HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs\.udt HKEY_USERS\S-1-5-21-1334085195-1259877840-1684446657-1000\Software\Classes\.udt HKEY_USERS\S-1-5-21-1334085195-1259877840-1684446657-1000\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.udt HKEY_USERS\S-1-5-21-1334085195-1259877840-1684446657-1000\Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs\.udt HKEY_USERS\S-1-5-21-1334085195-1259877840-1684446657-1000_Classes\.udt HKEY_CLASSES_ROOT\.udt HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.udt Apparently, .udt file types have significance outside of NoteTab 7's Dictionary folder. I did find on Google that "UDT" can mean "User Defined Type", but it's difficult for me to accept that just binding .udt to NoteTab 7 generated all 9 regustrt keys shown above. Therefore, I am left with this question which I've been unable to solve: How do I restore the .udt file extension to "Unknown application" in \Default Programs\Set Associations\ ? Kind regards, RetiredEngineer

Using regedit, navigate to the merged HKEY_CLASSES_ROOT hive. 1. In the left pane scroll down to your .udt extension, highlight it and make a note of the value of (Default) in the right pane. This is the file type, so scroll further down past the extensions to this file type (it may not exist). If it does, right-click it and choose Delete (this will also remove all sub-keys). 2. Scroll back up to the .udt extension, right-click it and delete it. Close regedit, you may have to restart the computer. To be safe, I suggest you create a system restore point first, just in case something goes wrong. 3. To understand extensions, file types and their associated programs, an easier to understand example of the above is a .txt file, which we all use in Notepad: Using a CMD prompt and typing assoc .txt will return txtfile (the file type). Typing ftype txtfile will return %SystemRoot%\System32\notepad.exe %1 (The full path of the program that opens when the file is double-clicked and %1 representing a passed parameter) Alternatively, look in the registry hive HKEY_CLASSES_ROOT for .txt and further down for txtfile. I hope this helps.

Using regedit, navigate to the merged HKEY_CLASSES_ROOT hive. 1. In the left pane scroll down to your .udt extension, highlight it and make a note of the value of (Default) in the right pane. This is the file type, so scroll further down past the extensions to this file type (it may not exist). If it does, right-click it and choose Delete (this will also remove all sub-keys). 2. Scroll back up to the .udt extension, right-click it and delete it. Close regedit, you may have to restart the computer. To be safe, I suggest you create a system restore point first, just in case something goes wrong. 3. To understand extensions, file types and their associated programs, an easier to understand example of the above is a .txt file, which we all use in Notepad: Using a CMD prompt and typing assoc .txt will return txtfile (the file type). Typing ftype txtfile will return %SystemRoot%\System32\notepad.exe %1 (The full path of the program that opens when the file is double-clicked and %1 representing a passed parameter) Alternatively, look in the registry hive HKEY_CLASSES_ROOT for .txt and further down for txtfile. I hope this helps.

BurrWalnut: I followed your instructions to the letter, but still have a nagging worry about the result not being what I expected. In \Default Programs\Set Associations\ I was expecting .udt to come back as an "Unknown Application", which is where it was before I assigned it to NoteTab. "Unknown Application" is a value many of my registry entries possess, such as .swf (Shockwave Flash Object) and .sys (System File). However, .udt has disappeared entirely from in between .udl (Microsoft Data Link) and .url (Internet Shortcut). Is not having a file extension listed the same as having it listed as an "Unknown Application?" Will my W7 system have any problems with this? Many thanks for your help so far.

The instructions I gave were to delete the association between the extension and its file type. It will not affect Windows 7 other than double-clicking a .udt file will produce an error. Generally .UDT (.DAT is another one) is an 'unofficial' extension used by many developers to store data for their programs.

Thanks for your insights.

Thanks for your insights.