I tried using the Set associations from Control Panel in windows 7, but it point me to another application , not the "unknown"...
You can copy the follow context to save as Default_DLL.reg file. This registry file will do the trick. I think you will also have the original "gears"for DLL files....
Windows Registry Editor Version 5.00
[-HKEY_CLASSES_ROOT\.dll]
[HKEY_CLASSES_ROOT\.dll]
@="dllfile"
"Content Type"="application/x-msdownload"
[HKEY_CLASSES_ROOT\.dll\PersistentHandler]
@="{098f2470-bae0-11cd-b579-08002b30bfeb}"
[-HKEY_CLASSES_ROOT\dllfile]
[HKEY_CLASSES_ROOT\dllfile]
"AlwaysShowExt"=""
@="Application Extension"
"EditFlags"=hex:01,00,00,00
"FriendlyTypeName"="@shell32.dll,-10146"
"NoOpen"=""
[HKEY_CLASSES_ROOT\dllfile\DefaultIcon]
@="imageres.dll,-67"
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.dll]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.dll]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.dll\OpenWithList]
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.dll\OpenWithProgids]
"dllfile"=hex(0):
