Apple apps such as Photos, iMovie, Safari, Keynote and QuickTime Player work with many media formats. Some apps prefer specific formats, but most formats work in most apps without additional software. Older or specialised formats may not work as expected if your device, its operating system or the app you’re using doesn’t support the format.
Install all available software updates for your Mac and software updates for your iPhone or iPad. After updating, if you don’t have an app that supports the older or specialised media format used by the file, search the internet or App Store for an app that does. If you know the name of the media format, include it in your search.
On Mac, you may be able to identify the media format by selecting the file in the Finder, then pressing Command-I to open an Info window. The format may appear after the label “Kind”, such as “Kind: Matroska Video File”. You can also search by the filename extension, which is the letters shown at the end of the file’s name, such as .avi, .wmv or .mkv.