This is because who every built the deployment package in the Adobe Customization Wizard set that preference in the MST to prevent users from opening non-PDF file and changing that setting.
I built the package myself with the Customization Wizard and deliberately did not disable the opening of non-PDF files, so that is not it. I found that we had a group policy setting in place prior to the rollout of Acrobat 11, however. It set the keys "HKCU\Software\Adobe\Adobe Acrobat\11.0\Originals\bAllowOpenFile" and "HKCU\Software\Adobe\Adobe Acrobat\11.0\Originals\bSecureOpenFile". These 2 settings will both modify the setting and also not allow the user from modifying this setting.
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try the registry keys:
iFileAttachmentPerms DWORD = 2
iUnlistedAttachmentTypePerm DWORD = 2
Or you can delet the Keys.
We will thanks Adobe Support about that they can't help .