fix: accept files exactly at fileSize limit

[abhu85]

Summary

Fixes #1348

Busboy emits the 'limit' event when fileSize equals limits.fileSize, causing multer to reject files that are exactly at the configured limit. This is an off-by-one error - files at the limit should be accepted; only files exceeding the limit should be rejected.

Root Cause

In busboy's multipart.js:

if (fileSize === fileSizeLimit) {
  this._fileStream.emit('limit');
  // ...
}

The 'limit' event fires when the file size equals the limit, not when it exceeds it. This is arguably correct behavior for busboy (it can't know if more bytes are coming), but multer should handle this case gracefully.

Solution

Pass fileSize + 1 to busboy so that files at the exact limit don't trigger the limit event. Files 1 byte over the configured limit will still trigger the event correctly.

var busboyLimits = limits
if (limits && typeof limits.fileSize === 'number') {
  busboyLimits = Object.assign({}, limits, { fileSize: limits.fileSize + 1 })
}

Test Plan

• Added test: file exactly at limit is accepted
• Added test: file 1 byte over limit is rejected
• Added test: empty file accepted when limit is 0
• Added test: non-empty file rejected when limit is 0
• All existing tests pass (76 total)