You first have to change the security settings and allow downloads from anywhere as opposed to only from the Apple Store.
Okay... Now I understand what you are referring to here.
Many users will find that daunting, if not dangerous.
Apple seems to consider it a security issue. Since I rarely download new applications without researching them, I had forgotten that I'd turned this setting off when I could no longer remember the temporary override keyboard shortcut under Yosemite.
It seems crazy not to provide a way to play AVI files without circumventing/compromising security?
AVI is an open file container originally developed/adopted by Microsoft who later abandoned it (about a dozen years ago) in favor of its more modern Windows Media product. The container is still supported by QTX but files are only playable if encoded with Motion JPEG video with almost any form of PCM encoded audio. Unfortunately, the QTX embedded AVFoundation structure is not programmed to play any other AV content even if the codecs are QTX supported. With the release of QT 7 in April of 2005, Apple made the decision to adopt MPEG-4 audio and video content as the "default" format for Mac operating systems and has since been busy degrading/removing support for third-party proprietary and/or hybrid, as well as, Apple's own "legacy" codecs. (Apple programmers even forgot to support its own "Pro" codecs in the initial release of Mavericks which created quite a stir among professional and "prosumer" users.) Basically, like most purveyors of video media, Apple is consolidating codec use to a small number of easily supported compression formats having the widest possible range of targeted uses—in effect forcing users to employ other players like VLC that continue to support older, third-party, non-proprietary media formats by embedding the codec support directly in the player. Apple appears to feel that if you opt to use third-party or unsupported codecs in supported files containers, then you should use alternative players for their playback and, thus, Apple avoids any potential copyright and/or codec support issues .
