Express API file uploads error: ECONNABORTED

I've got an API built in Express, which I'm testing with Jest. The API is working just fine when I test with Postman, but when I run my test suite I usually get an ECONNABORTED error on my file uploads using .attach(). (Occasionally, it works just fine.) I'm using a suite of middleware to download an array of images referenced by URL, followed by any images passed to the endpoint via form upload. After the images are saved, I'm using a third piece of middleware to resize the images and save them in a subdirectory. Since the error only happens in Jest, it seems like that's where the error would lie, but I'm also not positive I'm handling errors with Multer correctly.

// endpoint.js

const storage = multer.diskStorage({
  destination: async (req, file, cb) => {
    // Check for product's existence
    if (product) {
      // Make the directory if necessary
      cb(null, /* location */)
    } else {
      cb(404, null)
    }
  },
  filename: async (req, file, cb) => {
    cb(
      null, /* location */}`
    )
  }
})

const upload = multer({ storage }).array("files")

router.post(
  "/:id",
  async (req, res, next) => {
    // Download images from URLs if provided
    next()
  },
  (req, res, next) => {
    upload(req, res, err => {
      // Handle errors (e.g. return res.status(404).json({ message: "Product not found" })
    })
    next()
  },
  async (req, res) => {
    // resize and save to subdirectory
  }
)

// endpoint.test.js

describe("/endpoint", () => {
  describe("POST", () => {
    it("saves uploaded images to the server", async () => {
      const response = await request(app)
        .post(`/endpoint/${id}`)
        .attach("files", "assets/img-test-1.jpg")
        .attach("files", "assets/img-test-2.jpg")
        .set("Content-Type", "multipart/form-data")

      expect(response.status).toEqual(201)
    }, 30000)

  ...

})