I installed nextcloud 29.0.4，but i found that i cannot preview my mp4 file , When I click on the MP4 file, it downloads the MP4 file directly instead of playing it，how can i solve this problem，thank you !
I think you’re addressing two different topic in one, the missing preview and an issue with the video streaming. I have in mind that both topics have been discussed in this forum a couple of days ago, so you should find the answers by searching the forum.
Can you use a test video e.g. mov_bbb.mp4 (774 kb, peach.blender.org, about/license, Wikipedia). Post screenshots, logs, test with browser dev tools F12 and network analysis.
You can test other versions from here and here.
For bigger files you can download directly in Nextcloud with the app Transfer then you must not download to client and reupload to your Nextcloud. Works for me in Nextcloud 29.
I don’t (yet) have a preview image either. In addition, the file seems to be loaded completely into the browser first. This also takes some time in my example with a 60 MB file of “Big Buck Bunny”.