Strange Behavior with Multer

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Strange Behavior with Multer File Upload Limit and Filename Validation

I'm encountering a strange issue with the file upload logic using Multer in my Node.js project. The setup is as follows:

• File Size Limit: 5MB
• Allowed File Names: ml-amz.csv and ml-wal.csv
• Max Files: 3 files

The problem occurs when I try to upload a file with the incorrect filename (e.g., AMZ.csv instead of ml-amz.csv) and the file size exceeds the 5MB limit (e.g., 50MB).

Expected Behavior:

When uploading a file that exceeds the size limit and has an incorrect filename, I expect the following:

• The filename check should fail, triggering an error with the message: "Invalid file name. Allowed: 'ml-amz.csv' or 'ml-wal.csv'".
• The file should not continue processing further, and I should not see any additional processing issues (like being stuck or delayed).

Actual Behavior:

1. When I upload a file that exceeds the size limit (e.g., 50MB) with an incorrect filename (e.g., AMZ.csv):

   • Instead of triggering the filename validation error (LIMIT_FILE_NAME), the request just hangs and does not return any error.
   • The process does not move forward, and I don’t see any clear error message for either file size or filename validation.

2. When I upload a file that is within the size limit (e.g., less than 5MB) but has an incorrect filename:

   • The filename validation correctly triggers the LIMIT_FILE_NAME error with the appropriate error message: "Invalid file name. Allowed: 'ml-amz.csv' or 'ml-wal.csv'".

This issue seems to be occurring when the file size exceeds the limit, and it's not correctly handling the filename validation error first.

Code Snippet:

Here is the relevant Multer configuration code:

import multer from 'multer';

const MAX_FILE_SIZE = 5 * 1024 * 1024; // 5MB

const storage = multer.memoryStorage();

export const UploadLimitML = multer({
    storage,
    limits: {
        fileSize: MAX_FILE_SIZE, 
        files: 3
    },
    fileFilter: function (req, file, cb) {
        const fileName = file.originalname.toLowerCase(); 
        const allowedFileNames = ['ml-amz.csv', 'ml-wal.csv'];

        if (!allowedFileNames.includes(fileName)) {
            const error = new Error('Invalid file name. Allowed: "ml-amz.csv" or "ml-wal.csv"');
            error.code = 'LIMIT_FILE_NAME';
            return cb(error);
        }

        cb(null, true); 
    },
}).array("files", 3);

export function MulterMLErrors(err, req, res, next) {
    if (err instanceof multer.MulterError) {
        switch (err.code) {
            case 'LIMIT_FILE_SIZE':
                return res.status(400).send({ message: 'File size exceeds the allowed limit of 5MB.' });
            case 'LIMIT_FILE_COUNT':
                return res.status(400).send({ message: 'Only one file is allowed per request.' });
            default:
                return res.status(400).send({ message: 'An unknown upload error occurred.' });
        }
    }

    const allowedError = ['LIMIT_FILE_NAME'];
    if (allowedError.includes(err.code)) {
        return res.status(400).send({ message: err.message });
    }

    next();
}

Question:
Has anyone encountered a similar issue where neither file size validation (LIMIT_FILE_SIZE) nor filename validation (LIMIT_FILE_NAME) triggers? It seems that when the file size exceeds the limit and the filename is incorrect, the request just hangs without triggering any error. Is there a configuration issue or logic flaw here?

I would greatly appreciate any help on where I might be making an error in the logic or configuration. I’m not sure if this is a conflict in how Multer handles the validation or something else in my setup.

Any insights or suggestions would be greatly appreciated!