Low level MIME sniffing

[jimmywarting]

What is the problem this feature will solve?

This issue came up during a discussion related to fs.openAsBlob, where blob.type currently returns an empty string. I must agree that it would be nice if Node.js could automatically determine the MIME type of a given entity, whether through extension lookup or by inspecting "magic numbers".

Of course, there might be some cases where certain file extensions collide, but I believe it would be best to adhere to the IANA standards. After all, all browsers are capable of deducing certain information. Here's a straightforward example:

var root = await navigator.storage.getDirectory()
var fileHandle = await root.getFileHandle('image.png', { create: true })
var file = await fileHandle.getFile()
console.log(file.type) // Returns 'image/png'

I'm not sure about the exact mechanism behind it, but it simply works somehow.

I've come across some useful resources that might help address this issue:

• Define File's type better (getFile()) whatwg/fs#127
• Standardize extension to content type mapping? w3c/FileAPI#51
• WhatWG MIME Sniffing Specification

These resources could provide valuable insights and potential solutions for improving the MIME type detection in Node.js.

What is the feature you are proposing to solve the problem?

I'm unsure about the API's potential appearance, but it would be beneficial to explore various methods for detecting it. I'm also uncertain whether it should return an array of potential matches along with their respective scores and probabilities.

For instance:

x.identifyType('txt') // Returns 'text/plain'
x.identifyExtension('text/plain') // Returns 'txt'
x.sniffTypeFrom(blob || arrayBuffer || arrayBufferView) // Infers 'text/plain'
x.sniffExtensionFrom(blob || arrayBuffer || arrayBufferView) // Infers 'txt'
x.standardize('text/javascript1.2') // Converts to 'text/javascript' following the specification at https://mimesniff.spec.whatwg.org/#ref-for-javascript-mime-type
x.detectFormat(anything) // Returns { extension: 'txt', type: 'text/plain' }
x.lookupInSystemRegistry(anything) // Searches, for instance, the Windows system registry for installed applications
x.IANA...

Perhaps when searching for a path, it could consider both the file name extension and perform MIME sniffing.

Lastly:

blob = await openAsBlob('./image.png')
blob.type // Returns 'image/type'

Please note that the exact method names and functionality would depend on the implementation details of the API.

I think this would serve the community to have this built right in as it's something that is very commonly used.

[bnoordhuis]

whether through extension lookup or by inspecting "magic numbers"

Both have problems. Just looking at the extension is prone to false positives (e.g. GIF file with a .jpg extension), while sniffing the first few bytes is problematic with non-files like fifos (can't seek, can't rewind.)

Even determining if a file is text/plain is challenging if you don't know the encoding. A file consisting of the octets 00 00 00 20 is text/plain only when you know it's UTF32-BE, otherwise it's application/octet-stream.

https://mimesniff.spec.whatwg.org/ seems to be aimed at HTTP, not file systems, so I don't think it's of much use. I feel node shouldn't be in the business of making educated guesses, not when everyone else is making different educated guesses.

[anonrig]

For future reference, we have a fast mimesniff parser at Ada - https://github.com/ada-url/mimesniff

[Uzlopak]

A million years ago i programmed this

https://github.com/parallax/jsPDF/blob/5d09af9135a2fe049c7d3c8b95df280d22e4a6db/src/modules/addimage.js#L51

Is this something we should put in a transform stream?

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