If you want the result that .PPS, .PPSM and .PPSX files on webpages open in Show mode, and .PPT, PPTM and .PPTX on webpages open in Edit mode, you need to create separate MIME types:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\MIME\Database\Content Type\application/vnd.ms-powerpoint-slideshow]
"Extension"=".ppsx"
[HKEY_CLASSES_ROOT\MIME\Database\Content Type\application/vnd.ms-powerpoint-slideshow8]
"Extension"=".pps"
[HKEY_CLASSES_ROOT\.ppsx]
"Content Type"="application/vnd.ms-powerpoint-slideshow"
[HKEY_CLASSES_ROOT\.pps]
"Content Type"="application/vnd.ms-powerpoint-slideshow8"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Secure Mime Handlers]
"application/vnd.ms-powerpoint-slideshow"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Secure Mime Handlers]
"application/vnd.ms-powerpoint-slideshow8"=dword:00000001
[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Secure Mime Handlers]
"application/vnd.ms-powerpoint-slideshow"=dword:00000001
[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Secure Mime Handlers]
"application/vnd.ms-powerpoint-slideshow8"=dword:00000001
[HKEY_CLASSES_ROOT\PowerPoint.Show.12]
"PreferExecuteOnMismatch"=dword:00000001
"BrowserFlags"=dword:0000000a
[HKEY_CLASSES_ROOT\PowerPoint.Show.8]
"PreferExecuteOnMismatch"=dword:00000001
"BrowserFlags"=dword:0000000a
[HKEY_CLASSES_ROOT\PowerPoint.ShowMacroEnabled.12]
"PreferExecuteOnMismatch"=dword:00000001
"BrowserFlags"=dword:0000000a
If you need to do this on a server, please refer to
Register the 2007 Office system file format MIME types on servers
http://technet.microsoft.com/en-us/library/ee309278(v=office.12).aspx