Maybe it'll work this time?

main.js

@@ -14,6 +14,8 @@ import https from 'https';
 import { fileURLToPath } from 'url';
 import { dirname } from 'path';
 
+const request = require('request');
+
 
 // Load settings from config.json
 const __filename = fileURLToPath(import.meta.url);
@@ -121,6 +123,31 @@ function createOrJoinSMSRoom(tel, callback) {
     });
 }
 
+function getAndUploadFile(url, callback) {
+    logger.info("Downloading MMS media " + url);
+    // download
+    request({url, encoding: null}, (err, resp, buffer) => {
+        // upload
+        logger.info("Uploading MMS media to Matrix " + url);
+        client.uploadContent(buffer, {
+            onlyContentUri: true
+        }).then((res) => {
+            if (typeof callback == "function") {
+                callback(res);
+                logger.info("Media URI: " + res);
+            }
+        }).catch((err) => {
+            if (typeof callback == "function") {
+                callback(false);
+            }
+            if (err.data.error == "Unknown room") {
+                return;
+            }
+            logger.error(err);
+        });
+    });
+}
+
 /**
  * Send a message to a Matrix room.
  * @param {string} roomid the room to post the message in.
@@ -135,10 +162,11 @@ function sendMatrix(roomid, body, media, callback) {
     }
 
     if (typeof media == "string") {
+        getAndUploadFile(media, function (uri) {
             var content = {
-            body: media,
+                body: body,
                 msgtype: "m.file",
-            url: media
+                url: uri
             }
             client.sendEvent(roomid, "m.room.message", content, "").then((res) => {
                 if (typeof callback == "function") {
@@ -153,12 +181,14 @@ function sendMatrix(roomid, body, media, callback) {
                 }
                 logger.error(err);
             });
+        });
     } else if (Array.isArray(media)) {
         for (var i = 0; i < media.length; i++) {
+            getAndUploadFile(media[i], function (uri) {
                 var content = {
-                body: media[i],
+                    body: body,
                     msgtype: "m.file",
-                url: media[i]
+                    url: uri
                 }
                 client.sendEvent(roomid, "m.room.message", content, "").then((res) => {
                     if (typeof callback == "function") {
@@ -173,6 +203,7 @@ function sendMatrix(roomid, body, media, callback) {
                     }
                     logger.error(err);
                 });
+            });
         }
     }