Jodit sends files as files[0], files[1], ... (from filesVariableName(i)
returning `files[${i}]`), not files[] as previously assumed. The buildData
callback only matched "files[]" so the actual file field was passed
through as-is, triggering MulterError: Unexpected field because backend
multer.single("file") only accepts a field named "file".

Fix buildData to:
- Match files[0], files[1], ... pattern and convert first file to "file"
- Drop extra files (multer.single only accepts one)
- Drop Jodit-internal fields (source, extension, mimetype) backend ignores

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>