How to access uploaded file from multer?

Im able to upload an image to S3. Now, if the file selected is .gif, I want to be able to convert the .gif file to .mp4 and upload the converted file to S3. I am able to convert a .gif to .mp4 with ffmpeg only if I give the path of the file. How do I access the uploaded file from Multer? Below is my code :

var express = require('express');
var bodyParser = require('body-parser');
var app = express();
var aws = require('aws-sdk');
var multer = require('multer');
var multerS3 = require('multer-s3');
var s3 = new aws.S3();
var ffmpeg = require('fluent-ffmpeg');


var upload = multer({
    storage: multerS3({
        s3: s3,
        bucket: 'myBucket',
        key: function (req, file, cb) {
            console.log(file);
            var extension = file.originalname.substring(file.originalname.lastIndexOf('.')+1).toLowerCase();

                if(extension=="gif"){
                console.log("Uploaded a .gif file");

                ffmpeg(file) //THIS IS NOT WORKING
                    .setFfmpegPath("C:\\ffmpeg\\bin\\ffmpeg.exe")
                      .output('./outputs/2.mp4')    //TRYING TO UPLOAD LOCALLY, WHICH FAILS
                      .on('end', function() {
                        console.log('Finished processing');
                      })
                      .run();
            }

            cb(null, filename);
        }
    })
});

I'm trying to access the uploaded file like this: ffmpeg(file) since file is an argument passed in the multer function.

My form :

<form action="/upload" method="post"  enctype="multipart/form-data">
    <input type="file" name="image"> <br />
    <input type="submit" value="Upload">
</form>

Please help. Many thanks.

via Somename