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 specialized formats might 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 specialized 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 might 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 might 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.