mp4 "can’t be opened because it is from an unidentified developer."

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Level 2

356 points

mp4 "can’t be opened because it is from an unidentified developer."

I have a bunch of different mp4 files (some downloaded, some older content I had on my drive but haven't used in a while) that give me the "[file.mp4] can’t be opened because it is from an unidentified developer" error. This only happens with some files.

However, it only happens under the following circumstances:

If the default application is either QuickTime Player 7 or the regular QuickTime Player that's installed by High Sierra, and

a) I double-click the file, or

b) if I drag-and drop the file onto the QuickTime Player icon (either version) in the Dock or Application Switcher. I also get the error if I drag-and-drop the file onto VLC, either the icon or the Playlist window.

In either case, I get a "verifying" dialog first, then it fails.

I do NOT get the error if:

a) within either version of QuickTime Player I choose Open and navigate to the file and open it within the Open dialog, or

b) I change the Open With application to VLC and double click the file, or

c) with the default application set to VLC, I drag/drop the file to either QuickTime Player or to VLC (i.e., same as "b" above, but with the default application set to VLC.

In the latter circumstances (where I can open the file) I do not see the "Verifying" dialog.

There may be other combinations but I haven't tried them.

The Security and Privacy PP says that the mp4 was blocked from opening because it is not from an identified developer. I realize that I can unlock the preference pane and allow it to be opened, but I'd like to know why it's blocking a file, rather than an application, and under odd circumstances.

I posted this as a High Sierra question rather then QuickTime because of how VLC factors into it.

Any ideas?

Posted on Jan 20, 2019 4:11 PM

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Answer 1

Best reply

BDAqua

Level 10

239,279 points

Jan 20, 2019 4:31 PM in response to Scott_R

Whew, never heard of anything but an executable file reporting that!?

Aha, my first line give me a logical clue, I bet they're set to Open with... something wrong...

This answer applies if you set the app for that particular document, not for all documents of that type (e.g. by using Get Info on the document, and changing the "Open with" pop-up menu, but notclicking the "Change All" button).

There are two critical pieces here. First, if you set a document to open in a particular app, what it actually does is attach some metadata to the file (think of it like sticking a post-it note onto the file) that says to open it with that particular app. Second, if a document is quarantined (because it was downloaded from the internet, or created by a sandboxed app, or some other reasons), it's considered untrusted, and the gatekeeper security policy will be applied to anything like executable in the file.

These two features can interact in an unfortunate way: that "open with" note is an instruction about what to do, and therefore (sort of) executable code, and therefore the gatekeeper security policy applies to it (even though you created the note, it's "part of" the untrusted file, and therefore untrusted). Thus, double-clicking a document (with "open with" metadata) can get you the untrusted app warning/error.

Fortunately, as long as you want all files of that type (.cs in this case) to open in the same app, there's a solution: in the Info window, Open with section, select the app you want (e.g. Visual Studio Code), and then click "Change all". This removes the metadata from the file, and instead makes an entry in your Launch Services preferences saying that you prefer to have files of that type open in that app. Since the setting is now a personal setting rather than a note attached to an untrusted file, the problem goes away.

https://apple.stackexchange.com/questions/251205/why-is-osx-telling-me-i-cant-open-a-document-from-an-unidentified-developer

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Answer 2

BDAqua

Level 10

239,279 points

Jan 20, 2019 4:56 PM in response to Scott_R

Thanks, I wonder if the Launch DB is borked?

You can also open files by dragging them to the icon of another App that can open them.

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Answer 3

Scott_R Author

Level 2

356 points

Jan 20, 2019 4:50 PM in response to BDAqua

I'd had all mp4s set to open in VLC. But I'd wanted to compare the quality of two versions of the same video (one recently created by me with FFWorks, one much older and I don't recall how I created it) so I changed the Open With for both (separately) to open in QuickTime Player (since, AFAIK, you can't play two videos at the same time with VLC without some extra work). The older version gave me the error.

Similarly, a downloaded mp4 (I don't know how it was created) had VLC set as the default at the time of downloading (presumably, because VLC had been set as the default for all mp4s), but II changed its Open with... setting.

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