fix: do not consume maxCount for files skipped by fileFilter

[Sagargupta16]

Description

wrappedFileFilter decrements the field's remaining-count before the user's fileFilter runs:

filesLeft[file.fieldname] -= 1
fileFilter(req, file, cb)

So a file the user skips with cb(null, false) still consumes a slot. With .array('docs', 1), uploading a skipped file followed by an accepted file on the same field makes the second file fail with LIMIT_UNEXPECTED_FILE — even though zero files were stored. Per the README, cb(null, false) means the file is silently skipped (never added to req.files), so it must not count against maxCount. Fixes #1419.

Fix

Move the decrement inside the fileFilter callback and only decrement when the file is actually accepted:

fileFilter(req, file, function (err, includeFile) {
  if (!err && includeFile) filesLeft[file.fieldname] -= 1
  cb(err, includeFile)
})

This routes all callers (.single / .array / .fields) through one guard. Truly unexpected fields and accepted files beyond maxCount still error as before.

Test

Added a regression test in test/file-filter.js: a .array('docs', 1) upload where the filter skips the first same-field file and accepts the second — asserts exactly one file (tiny1.dat) is stored with no error. The test fails on main without the fix (LIMIT_UNEXPECTED_FILE) and passes with it; existing fileFilter tests remain green.