MP3 decoding, video playback and export in Live 10.1.2 and later (macOS)

Live 10.1.1 and older used QTKit (Quicktime). As part of the transition from 32-bit to 64-bit technology in macOS 10.15 Catalina, Apple has dropped support entirely for QTKit. 

AVFoundation

As of Live 10.1.2, Live uses AVFoundation for MP3 decoding, video playback, and video export on macOS. AVFoundation is only used for MP3 and M4A audio files. The compressed audio file decoding experience should be unchanged, while video playback and export is significantly faster. 

MP3 export 

MP3 export/encoding uses the LAME MP3 Encoder and is unaffected by this change.

Unsupported formats

Videos or files created using older formats or codecs that rely on QuickTime 7 are not longer supported as of macOS 10.15 or in AVFoundation. See this list of compatible formats on macOS Catalina.

Note: Although listed in the compatible format list linked above, AVFoundation itself does not support image files like jpeg, gif, png. Therefore, it's no longer possible to add image files to Live sets. However some motion JPEG files are supported.

On macOS Mojave (10.14) or earlier, unsupported files can be converted into a supported format by opening them in QuickTime Player. Apple has published an article with instructions: Converting legacy media files.

As of macOS 10.15 Catalina, you need to use a computer with an older macOS to convert files, as unsupported formats cannot be opened.