When you open an email attachment, Outlook displays the Opening Mail Attachment dialog with Open and Save buttons for some file types.

When the attachment is a Level2 attachment type, the "Always ask before opening" option is always disabled as part of Outlook's increased security. See "Block Additional File Types in Outlook" for information to block or unblock file types.

When you use Windows7 or newer, the Always ask before opening option may be disabled for file types normally considered safe.

When this happens, you need to always respond to the dialog and choose whether to open or save the attachment. Whether its enabled or disabled is controlled by the UAC (user access control) and if you run Outlook as an administrator.

EditFlags Registry Key

The EditFlags binary value is the key. In Outlook 2007 and 2010, the EditFlags value may not exist but creating it with 00 00 01 00 data gets rid of the dialog and opens the message.

This example is for GIF extension:

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\giffile]
@="GIF Image"
"ImageOptionFlags"=dword:00000003
"EditFlags"=hex:00,00,01,00

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\giffile\
EditFlags name value contains binary data of
00 00 00 00 = display 'always ask' dialog
00 00 01 00 = always open, no dialog

ImageOptionFlags controls whether its opened or saved.

Word 2007 DOCX extension is
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Word.Document.12

DOC extension:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Word.Document.8

Excel's .xslx extension:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Excel.Sheet.12

PowerPoint's .pptx extension:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\PowerPoint.Show.12

Adobe PDF
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\AcroExch.Document

Find the Keys

In some cases, you can find the key for each extension by looking at the .extension key. Under the extension type, you may see subkeys for PersistentHandler and ShellEx along with one identifying the file type by name. Look for that name under Classes. Ie, .docx has a subkey for Word.Document.12:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.docx\Word.Document.12
so the key you'll add an EditFlags value to is:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Word.Document.12

Doc is HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.doc\Word.Document.8
so the key you'll add an EditFlags value to is:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Word.Document.8

Screenshots:
Registry for \.docx\Word.Document.12

Registry for Classes\Word.Document.12

Note that not all of the .extension keys have a key that identifies the file type by name.

Do it for me

If you aren't into editing the registry yourself, this registry file will add or change the edit flags setting for Word (docx, doc), Excel (xlxs, xls) and Powerpoint (ppts, ppt) files.

EditFlags for Word, Excel, and Powerpoint

IE's "Always Ask..." dialog

In Windows XP, the "Always ask" dialog is shared between IE and Outlook. If you set it to always open in one application, it will always open in the other.

In Vista the "Always ask" settings are stored in different keys. The settings for IE are stored under the following key. Delete the values for the file types you want to display the dialog.

HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\AttachmentExecute\{0002DF01-0000-0000-C000-000000000046}