We're hearing from many users: Outlook always prompts for permission to open attachments and the checkbox for "Always ask before opening this type of file" is grayed out. This is due to User Access Control (UAC) restrictions in Windows.

There are two ways you can address this:

1: Run Outlook as Administrator and open the attachments, removing the check from "Always ask before opening this type of file" box. After you set the desired file types to open without asking, you can run Outlook normally.

2: If you know the file types and are comfortable editing the registry or want to change it via policy, you'll need to identify the file type in HKEY_CLASSES_ROOT or (buried under) HKEY_LOCAL_MACHINE if Windows 10 64bit and Office 2016 32bit. After locating the key, create or change the EditFlags Binary value to 00 00 01 00.

Run as Administrator

1. Hold the Shift key and right click on the shortcut you use to open Outlook.
2. Choose Run as Administrator.
   

Edit the Registry

If you are using Windows 7 (and newer) with Office in the same bitness (both Office and Windows are 32 bit, or both are 64bit), the path is under HKEY_CLASSES_ROOT.

DOC: HKEY_CLASSES_ROOT\Word.Document.8
DOCX: HKEY_CLASSES_ROOT\Word.Document.12
PDF: HKEY_CLASSES_ROOT\AcroExch.Document
XLSX: HKEY_CLASSES_ROOT\Excel.Sheet.12
XLS: HKEY_CLASSES_ROOT\Excel.Sheet.8
PPT: HKEY_CLASSES_ROOT\PowerPoint.Show.8
PPTX: HKEY_CLASSES_ROOT\PowerPoint.Show.12
HTML: HKEY_CLASSES_ROOT\htmlfile

EditFlags: 00 00 01 00

To always ask before opening, either delete the EditFlags value or change the value to 00 00 00 00

If you are using 32-bit Outlook on Windows 64-bit, the registry keys are now stored in a (crazy-long) path under HKEY_LOCAL_MACHINE. This is the path for 64-bit Windows and 32-bit Office:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\ClickToRun\REGISTRY\MACHINE\Software\Classes\

DOC: Word.Document.8
DOCX: Word.Document.12
PDF: AcroExch.Document
XLSX: Excel.Sheet.12
XLS: Excel.Sheet.8
PPT: PowerPoint.Show.8
PPTX: PowerPoint.Show.12

EditFlags:  00 00 01 00 (Binary Value)

If you have more than one PDF program installed, you will need to edit the keys for each PDF program.

Outlook 2016 32-bit on Windows 10 64-bit:

See Restore (or Hide) the Open Save Dialog if you need instructions for older versions of Windows.

Identify keys for other file types

To identify other file types, look at the extension's key in the registry - most (if not all) will include a more descriptive name. Locate this name under HKEY_CLASSES_ROOT and change it's EditFlags value.

For example, this is the key for the .pptx extension (shown in the screenshots):
HKEY_CLASSES_ROOT\.ppt\PowerPoint.Show.12
So you would change the EditFlags value at HKEY_CLASSES_ROOT\PowerPoint.Show.12

The locate the key by that name. Double click on the EditFlags key to open it for editing.

00 00 00 00 displays the Open or Save dialog
00 00 01 00 opens the file.

Do it for me

If you don't want to edit the registry, you can use this Registry file. It contains the EditFlag for Word, Excel, and PowerPoint listed above plus htmlfile.

Edit Flags registry keys 32-bit Outlook on 64-bit Windows

Download the file and double click to add the keys to the registry. Note that some browsers will save it with the .txt extension. You'll need to change the extension to .reg to use.