Going from Outlook 2010 to 2016, I’m finding a lot of new features and changes that need to be fine tuned for my environment. One of these is the ‘Opening Mail Attachment’ prompt that says ‘You should only open attachments from a trustworthy source.’. The tickbox to ‘Always ask before opening this type of file’ is greyed out, which would help a single user avoid the prompt, but also doesn’t help in an enterprise setting.
There are some guides online already that advise to open Outlook as an administrator, tick the box, and you’re set. However, I couldn’t find anything on how to automate this or roll out the setting.
After using Procmon, it appears that each file type is checked against the registry hive of HKEY_LOCAL_MACHINE\SOFTWARE\Classes\ – The file will be recognised as a certain class. Sometimes this is obvious where the class is named the same as the file extension, but usually it’s something more descriptive.
For example, a .bat file is under batfile, and known as a Windows Batch File. It also makes sense that the tickbox is greyed out, as a user will normally not be able to change HKEY_LOCAL_MACHINE settings without administrator privilidges.
Once you know what Class you’re looking at, there should be an ‘EditFlags’ registry key. From what I’ve seen, this can be either REG_BINARY, REG_DWORD. It appears that the default REG_BINARY value is 00 00 00 00, and the DWORD is 0x00000000. Changing the third set of 00’s to 01, or the third 0 in the set of 8 digits is what changes for that Class when the tick is removed from ‘Always ask before opening this type of file’.
For me, pushing out these settings below for .NRL files avoided the prompt for all users:
For whatever app you’re trying to do this for, I’d recommend using Procmon to check what changes when removing the tickbox, but at least with this information you should be able to narrow down what you’re searching for.
Update 12th February 2019:
Brian points out below that this registry value appears to have changed in the latest version of Office 365 CTR:
I haven’t verified this personally but Brian seems like a trustworthy person :)