Windows 7 How can I change .udt file extension back to "Unknown application" ?

[RetiredEngineer]

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
  
  
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  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

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