As far as I understand, I have to pickup the mimeapps.list from 18.04 and put it in place of /etc/xdg/xdg-lubuntu/mimeapps.list in lubuntu-default-settings. obviously along with some edits. Am I right?
I don't think is the best strategy. Between 18.04 and 19.04 there are a lot of different programs. I think the best strategies is to look if all the mimetypes of 18.04 are present and the ones that are not add. It with the new program
Downloaded a sample image, pdf, and two videos. Opened in pcmanfm-qt and double clicked and they opened in lximage, qpdfview, and vlc, respectively. Then opened a terminal and used xdg-open with the same results except on one of the two videos, a webm, which opened in firefox. I think video/webm is the right mimetype from what I can tell if you want to add that on as an additional plus, but looks like the solution as intended is effective. Good job, @The_LoudSpeaker!