Discord.js bot errors involving embedded images - bots

Question

So I programmed a discord bot to, on command, send a randomly selected gif from a folder in an embedded message. It worked fine until I started trying to host the bot from Heroku. Now the command does nothing but displays an error in the log saying something like "could not find file extension", "deprecation error", or, just now, "unhandled promise rejection". My main issue is that I'm very new to programming in general, so I have no clue what exactly my problem is, let alone how to resolve it.
This is the current code I have for this particular section:
case '//command':
const Embed = new MessageEmbed()
var number = 43;
var imageNumber = Math.floor (Math.random()*(number-1+1))+1
Embed.setTitle("//message")
Embed.setDescription("//message")
Embed.attachFiles({files: ["./images/" + imageNumber + ".gif"]})
Embed.setImage("attachment://" + imageNumber + ".gif")
Embed.setColor(0x9B00C3)
msg.channel.send(Embed)
break;
Edit: as requested, here is the error message that appears:
2020-07-11T17:23:22.063132+00:00 app[web.1]: (node:23) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'path' of undefined
2020-07-11T17:23:22.063143+00:00 app[web.1]: at findName (/app/node_modules/discord.js/src/structures/APIMessage.js:273:17)
2020-07-11T17:23:22.063144+00:00 app[web.1]: at Function.resolveFile (/app/node_modules/discord.js/src/structures/APIMessage.js:289:31)
2020-07-11T17:23:22.063145+00:00 app[web.1]: at /app/node_modules/discord.js/src/structures/APIMessage.js:224:72
2020-07-11T17:23:22.063147+00:00 app[web.1]: at Array.map (<anonymous>)
2020-07-11T17:23:22.063147+00:00 app[web.1]: at APIMessage.resolveFiles (/app/node_modules/discord.js/src/structures/APIMessage.js:224:46)
2020-07-11T17:23:22.063148+00:00 app[web.1]: at TextChannel.send (/app/node_modules/discord.js/src/structures/interfaces/TextBasedChannel.js:172:46)
2020-07-11T17:23:22.063149+00:00 app[web.1]: at Client.<anonymous> (/app/index.js:76:25)
2020-07-11T17:23:22.063149+00:00 app[web.1]: at Client.emit (events.js:315:20)
2020-07-11T17:23:22.063149+00:00 app[web.1]: at MessageCreateAction.handle (/app/node_modules/discord.js/src/client/actions/MessageCreate.js:31:14)
2020-07-11T17:23:22.063150+00:00 app[web.1]: at Object.module.exports [as MESSAGE_CREATE] (/app/node_modules/discord.js/src/client/websocket/handlers/MESSAGE_CREATE.js:4:32)
2020-07-11T17:23:22.063226+00:00 app[web.1]: (node:23) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
2020-07-11T17:23:22.063337+00:00 app[web.1]: (node:23) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.