Understanding Mac File Extensions

Need to convert a jpg image to a jpeg? Or an mov video to a mp4? Chances are you don't need to convert anything at all. You just need to change the filename. File extensions on a Mac are somewhat optional and often interchangeable. A jpg is the same as a jpeg file. Video files extensions aren't as important as the contents of the file as video file are really just containers.
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Video Transcript: Hi, this is Gary with MacMost.com. MacMost is brought to by the more than 300 people who contribute to the MacMost Patreon Campaign. Find out how you can become a part of that at macmost.com/patreon.
So let's talk about file extensions. File extensions are the letters, usually about three or four letters, after the dot or period in the filename. For instance, .docx, .jpg, and .mp4. These are file extensions. If you don't see them here you can go to Finder, Preferences and under Advanced you can check Show all Filename extensions. You see these here, despite the fact that I have that turned off, because I've used those filename extensions. I've used them when I've renamed or saved a file and Mac OS is smart enough to know to display them. As a matter of fact if you select a file, do Command I to get info on it, you'll see a Hide Extension checkbox. That's Off here meaning it's going to show that extension for this file no matter what I have set in Preferences.
So what about these extensions. You've got an extension like jpg. Well sometimes you don't see jpg. You'll see jpeg. Which one is correct? Well both are fine. You can go back and forth between either one of those. On your Mac it really doesn't matter. Mac is going to understand either one. jpg I can double click and it will open up in Preview and knows it's a jpg file. I can do Command I in Preview and it shows Document Type, jpg image. So it knows this from inside the image what type it is. The extension is just a little convenience. So I can change that.
Somebody would ask how do you convert from jpg from jpeg. You don't convert. You just change the file name. As a matter of fact Mac OS doesn't even complain when you switch between those two because it knows they're the same thing. I double click on it and it opens up in Preview as before. The only reason you would want to change the name is because sometimes different services or websites ask you submit something and they want it submitted as jpg and they won't accept say the format with the e in it. Maybe you have an old jpg image that has an e. So you can just take the e out and you're fine.
You can even try to fool Mac OS and it won't work. I can, for instance, take away the extension completely and it's find with that. Still opens up in Preview. Still knows it's a jpg. I can change to something like xxx and it doesn't really care about that. It still knows it's a jpg image. I can even go and try to fool it and say it's not a jpg image, it's a tif which is an older image format. Now here it will complain about changing the file format or the extension. But when I go to open it up it still opens it up. It goes and looks at it and says yah, okay it's an image. Send it to Preview. Preview says, oh this is actually a jpg image so I'll ignore this here. So you could do that.
Now you normally wouldn't want to do that with a third party app or some other apps. For instance, the docx format is a Microsoft word document. So if I double click on that it will open up Microsoft Word. But if I were to change it to something, like I would change it to jpg, then it thinks it's an image and changes the icon even. If I try to open it up it's going to say well let's let Preview handle that and Preview says I don't know what this is so you get an error. So you can't just change it. It doesn't ignore the file extension altogether. There's certain things you can change between like jpg and jpeg with an e.
Videos are really interesting because videos have different extensions like MP4, MOV, or M4B. These are called Containers. The types of files that they are, containers. They contain content. So it's not really a MOV video. It's a MOV container and inside of it is something. Which may actually be a MP4 video. So, you can sometimes, as a matter of fact quite often, change the file name very easily. 
It's similar to you could think of an envelope. An envelope, the physical one you send through the mail, could contain a letter. It could contain a check. It could contain a birthday card. It's still an envelope and you can't really tell what's inside by just looking at the envelope. Those file extensions indicate a container type and they contain similar things. Typically MOV containers can contain all sorts of old formats that used to be QuickTime formats including MP4 or mpeg4 files. But a MP4 extension should only contain things that are mpeg4 videos which are most videos. M4V is an even newer version of MP4, MP4 video.
So you can make changes. I can, for instance, double click on this file and it opens up in QuickTime player. But if I change the file extension to MOV then my Mac is fine with that. It will open it up in QuickTIme player just as before. If I change it to M4V it will also work just as before. Now when I do Command I inside of QuickTime player you can see it says the Format, H2 64, that's what is actually inside. That's the equivalent to say birthday card, check, or letter inside of the container. So all of those file containers, MOV, MP4, M4V, it doesn't matter when it comes to H2 64. It all fits.
The same is true for this which is an older type of Mpeg4 video. Not H2 64 but still pretty standard. It doesn't matter. It only really matters what the extension is, perhaps, with a much older video like a pre iPhone video where you may have recorded something with a camcorder or maybe still use an older piece of equipment that records in different things like an AVI format or something. Then the file extension may matter. But for most things that are MOV, MP4, or M4V you can change the file name if you want.
You shouldn't. You should use the file name generated by whatever app that created the file. But if you find the need to change it, for instance a website wants you to submit a MP4 and you have an MOV, just change the file extension and most likely it will work. So the basic thing is you don't stress about what the actual extension is and before you ever try to convert a video consider that the video may already be in the proper format. It's just the file extension that needs to be changed.
Understanding Mac File Extensions

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