how to see a preview of avi videos?

If you’re running Mac OS X 10.8.5 or earlier, this may require Perian, or another component, or may not be possible; AVI is a container, not a format.

If you’re running Mac OS X 10.9 or newer and it doesn’t happen already, you can’t. Only the codecs which are part of the OS will be checked by the Finder.

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how to see a preview of avi videos in the finder?

To add to what Niel said, the only AVI file format that is compatible with current versions of QTX and QL are AVI files containing Motion JPEG video with any compatible form of PCM encoded audio. If you must use AVI file containers and want to view them with QL, then encode the content as MJPEG/LPCM. Almost any form of PCM from u-law to AIFF or WAV should work. Foe example: