Invalid fileSize: send a JSON number

For POST /v1/upload/presigned-url and POST /v1/upload/confirm, the fileSize field must be a JSON number: the size of the file in bytes, with no quotes around it.

What you'll see

This is different from a file size mismatch after upload, where the file exists but the byte count you declared does not match what was stored.

Common mistakes

Quoted size in JSON ("1024" is a string)

{ "filename": "video.mp4", "contentType": "video/mp4", "fileSize": "1024" }

The value must be unquoted:

{ "filename": "video.mp4", "contentType": "video/mp4", "fileSize": 1024 }

Human-readable sizes

Values like "1.5MB", "1.46mbs", or "1,460,000 bytes" are not accepted. Compute the size in bytes (e.g. in JavaScript use file.size from a File object) and send that integer.

Building the body in JavaScript

JSON.stringify will serialize a number correctly as long as the value is actually a number:

const file = document.querySelector('input[type=file]').files[0]
await fetch('https://api.ffmpeg-micro.com/v1/upload/presigned-url', {
  method: 'POST',
  headers: {
    Authorization: 'Bearer YOUR_API_KEY',
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    filename: file.name,
    contentType: file.type,
    fileSize: file.size, // number — do not use String(file.size)
  }),
})

Use the same numeric byte value (and the filename returned from presign) when you call /v1/upload/confirm.

cURL

Use a numeric literal inside the JSON (no quotes around the digits):

curl -X POST "https://api.ffmpeg-micro.com/v1/upload/presigned-url" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"filename":"video.mp4","contentType":"video/mp4","fileSize":12345678}'

How to get file size in bytes

You need the exact byte length of the payload you will PUT to the presigned URL (same bytes at confirm). The API does not parse human strings like "2 MB"; your runtime must produce an integer.

Browser (file input) — File objects expose size in bytes:

const bytes = document.querySelector('input[type=file]').files[0].size

Node.js — length of the buffer you upload, or filesystem metadata:

import fs from 'fs'
const bytes = (await fs.promises.stat('/path/to/video.mp4')).size
// or: Buffer.byteLength(buf) if you already have the file in memory

Python

import os
bytes_count = os.path.getsize("/path/to/video.mp4")

macOS / Linux (terminal) — examples (flags differ by OS):

# macOS
stat -f%z /path/to/video.mp4

# Linux (GNU stat)
stat -c%s /path/to/video.mp4

n8n — See How to use ffmpeg with n8n cloud for downloadable workflows. The Extract Metadata sub-workflow sets fileSizeBytes from the binary buffer (buffer.length after this.helpers.getBinaryDataBuffer(...)), then passes that numeric field into the presign and confirm JSON bodies (unquoted). Import those workflows or copy the same pattern for your binary property.

Other stacks (Go os.Stat, .NET FileInfo.Length, etc.) follow the same idea: use the language’s file or buffer size in bytes, then put that value in JSON as a number.

Rules at a glance

More help