What's an application/octet-stream file?

Hello, I've had this problem with viewing expanded jpg's that up until now, I've been trying to get along with. On the website, milesplit.com, there's an option at the bottom of every picture, it gives the option saying to click here to view a larger version. After I click it, I get a small box saying "You have chosen to open..." and then a big string of letters and numbers for the file name. It continues "which is: application/octet-stream file (156 KB) from: d193frjqb908ar.cloudfront.net" And then asks what Firefox should do with it, either open or save it. From there, I always select Open with and click on Browse. The next screen is long and narrow and at the top it says to Choose Helper App. Then, it gives the application/octet-stream name, which happens to be the same no matter what picture you're looking at, and then it says Send this item to and gives (in my case) 6 apps to readily click on, with Firefox being at the top. As I always want Firefox to open it, I just click OK and then click OK again in the smaller box. And finally, the larger version of the picture is displayed in another tab. Now, I've been using this site on and off for at least 15 years and before a few years ago, I had no problems. So, is this a fixable problem, or do I have to keep on doing what I have been doing?