Formidable detects MIME-type according to the file extension and not by the real content

Support plan

• which support plan is this issue covered by? (e.g. Community, Sponsor, or
  Enterprise): Community
• is this issue currently blocking your project? (yes/no): no
• is this issue affecting a production system? (yes/no): yes

Context

• node version: 16.4.2
• module (formidable) version: 3.0.0-canary.20210428
• environment (e.g. node, browser, native, OS): Node.js
• used with (i.e. popular names of modules):
• any other relevant information:
  Formidable detects MIME-type according to the file extension and not by the real content. Which means that user can fake the file MIME by changing file's extension and as a result to upload to the server not allowed file types.

BTW, multer detects file's MIME not by the extension only.

What are you trying to achieve or the steps to reproduce?

1. Given JPG file tst.jpg
2. Rename it to tst.pdf
3. Set a break point inside of uploader.parse(req, async (err, fields, files) => {…}
4. Try to upload tst.pdf

uploader.parse(req, async (err, fields, files) => {
	if (err) {
		reject(err);
	} else {…}
});

What was the result you got?

mimetype = application/pdf

What result did you expect?

mimetype = image/jpeg
Since this file is actually JPG file but with a from extension.

Start by tracing how uploader.parse assigns the uploaded file's MIME type during multipart parsing. Reproduce the tst.jpg-to-tst.pdf case, then inspect existing upload-related tests and the MIME-detection path. Done means the reported MIME type reflects the file content rather than only its extension, with coverage for the reproduction.