IncorrectNumberOfFilesInPostRequest
AWS IncorrectNumberOfFilesInPostRequest (Incorrect Number Of Files In Post Request) means S3 POST upload requires exactly one file payload. In Amazon S3, this error returns HTTP 400.
Last reviewed: February 3, 2026|Editorial standard: source-backed technical guidance
What Does Incorrect Number Of Files In Post Request Mean?
S3 rejects the browser-based POST before object write begins, so upload UX breaks until the multipart form includes exactly one file part.
Common Causes
- -S3 POST form includes zero or multiple file parts instead of exactly one.
- -Multipart form builder duplicates file fields during request assembly.
- -Gateway or framework transformation alters form-data structure.
How to Fix Incorrect Number Of Files In Post Request
- 1Send exactly one file field in the S3 POST multipart form.
- 2Validate generated multipart body before request submission.
- 3Replay with a minimal signed POST form example.
Step-by-Step Diagnosis for Incorrect Number Of Files In Post Request
- 1Capture raw multipart payload and count file parts.
- 2Trace frontend/backend form assembly code for duplicate append paths.
- 3Compare generated POST fields with the signed policy contract.
Multipart Form Structure Validation
- -Inspect raw multipart boundaries and count file payload sections (example: drag-and-drop UI appends both original file input and preview blob).
- -Verify form builder does not emit empty placeholder file fields (example: hidden fallback input produces zero-byte second file part).
POST Policy and Field Contract Checks
- -Audit signed POST policy fields against submitted form keys (example: policy expects key/content-type but middleware injects extra file[] array field).
- -Replay with a minimal known-good one-file POST to isolate client assembly bugs (example: curl form succeeds while frontend multipart builder fails).
How to Verify the Fix
- -Re-run browser/form POST and confirm exactly one file part is accepted.
- -Validate signed policy fields and uploaded object metadata.
- -Ensure uploader no longer emits duplicate file parts.
How to Prevent Recurrence
- -Add multipart-form validation that enforces one file field.
- -Test upload builders for duplicate file append behavior.
- -Keep signed policy and form-generation logic in one module.
Pro Tip
- -define a strict typed upload contract where the server accepts a single canonical file field name and rejects array-style file inputs at the edge.
Decision Support
Compare Guide
HTTP 400 vs 422: Bad Request vs Unprocessable Content
Fix API payload issues faster by using 400 for malformed syntax and 422 for semantic validation failures, so clients correct format before business rules.
Playbook
CORS Error Fix Playbook (Preflight / Origin / Credentials)
Use this playbook to separate browser-enforced cross-origin policy failures from server-side CORS header and route defects and apply strict origin and credential controls safely.
Playbook
Validation Failure Playbook (400 / 422 / INVALID_ARGUMENT)
Use this playbook to separate malformed-request failures from semantic validation failures, then fix request contracts without broad server-side bypasses.
Official References
Provider Context
This guidance is specific to AWS services. Always validate implementation details against official provider documentation before deploying to production.