Office 2013 will not open downloaded files in Protected View

Question

I have long been using Office 2013 (on Windows 7). In the past, it has opened downloaded files in "Protected View", displaying the usual banner warning atop the opened file to indicate that the file is open in a restricted mode. This banner gave me the option to "unblock" the opened file from within Office (say, within Word). This is how I want Office to deal with downloaded files.

But, somehow, this behavior has changed in my Office installation. Now, Office will not open downloaded files at all. Instead, I receive an error message: "Word experienced an error trying to open the file," followed by the suggestion that the downloaded file may be corrupt. How can I fix this problem, so that the files open in Protected View?

The downloaded files aren't corrupt. And there are multiple ways around the problem, but none seem satisfactory. For example, I can "unblock" the file in the Windows 7 dialog box that displays the file's properties. That is the solution recommended in Office installation won't open files downloaded from a browser. But this is a cumbersome, file-by-file solution, and it leads to downloaded files being opened in a normal way, not in Protected View.

I could also change the Protected View settings in the Trust Center, as in MS office files downloaded from gmail are not opened. But this too would not do what I want: it would again lead to the files being opened as ordinary files, not in Protected View.

I want downloaded Office files to be opened in Protected View, and I want to be able to unblock them from within my Office applications. What settings can I change to restore this default behavior?