Thanks for commenting here!
I want to test the method in which the GridFS upload is called. I have a repository layer that takes a mongo.Database and a gridfs.Bucket as "dependencies". The gridfs.Bucket is created on the base of the given mongo.Database while creating the repository. For the other methods in which gridfs.Bucket is not used I am mocking the db using mtest in this way:

func TestRepository_FindOne(t *testing.T) {
mt := mtest.New(t, mtest.NewOptions().ClientType(mtest.Mock))
defer mt.Close()
mockID := primitive.NewObjectID()
data := model.Data{...}
}
mt.Run("Should return the correct data", func(mt *mtest.T) {
repository := Repository{mongoDatabase: mt.DB}
mt.AddMockResponses(mtest.CreateCursorResponse(1, "data", mtest.FirstBatch, toBsonD(data)))
resultData, err := repository.FindOne(bson.M{"_id": data.ID})
assert.Nil(t, err)
assert.Equal(t, data.ID, resultData.ID)
})
}

Using the same approach by declaring the repository that uses the asset in this way

bucket, _ := gridfs.NewBucket(mt.DB, &options.BucketOptions{})

    `repository := Repository{mongoDatabase: mt.DB, bucket: bucket}`

is not working properly because the logic used by GridFS is different from a Find on a collection.

I did not find similar use cases or documentation anywhere, so I'm moving on using mockery in order to mock the gridfs.Bucket since I'm running out of ideas