Discord Streamable Embed - Embed userscript for Streamable videos you can use until Discord fixes this issue.

discord.fixstreamableembed.user.js

// ==UserScript==
// @name        Discord Streamable Embed
// @description Embed userscript for Streamable videos you can use until Discord fixes this issue.
// @namespace   Discord Streamable Embed
// @require     http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js
// @match       *://discordapp.com/*
// 
// ==/UserScript==



(function($) {
    "use strict";

    $.fn.streamableMsg = function() {
        $("div[class$='embed-link']").each(
            function() {
                if ($(this).is("#streamableFixed") === false && $(this).html().indexOf("streamable") !== -1)
                {
                    $(this).attr('id', 'streamableFixed');
                    
                    var videoWidth = $(this).find("a[class*='embed-thumbnail']").width() * 5;
                    var videoHeight = $(this).find("a[class*='embed-thumbnail']").height() * 5;
                    var link = $(this).find("a[class*='embed-thumbnail']").attr('href').replace("https://streamable.com/", "https://streamable.com/t/") + "?r=t";
                    //alert(videoWidth + "x" + videoHeight + " for " + link);
                    //$(this).html('<video width="' + videoWidth + '" height="' + videoHeight + '" controls><source src="' + link + '" type="video/mp4"></video>');
                    $(this).html('<iframe width="' + videoWidth + '" height="' + videoHeight + '" src="' + link + '"></iframe>');
                }
            }
        );

        return this;
    };

    // Helper function for finding all elements matching selector affected by a mutation
    var mutationFind = function(mutation, selector) {
        var target = $(mutation.target),
            addedNodes = $(mutation.addedNodes);
        var mutated = target.add(addedNodes).filter(selector);
        var descendants = addedNodes.find(selector);
        var ancestors = target.parents(selector);
        return mutated.add(descendants).add(ancestors);
    };

    // Watch for new messages in chat
    new MutationObserver(function(mutations, observer) {
        mutations.forEach(function(mutation) {
            mutationFind(mutation, ".message").streamableMsg();
        });
    }).observe(document, {
        childList: true,
        subtree: true
    });
})(jQuery.noConflict(true));