expressjs/multer

File type shows correct on the frontend but it changes to application/octet-stream on the backend

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#1250 opened on Apr 6, 2024

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help wantedquestion

Description

I am uploading file from frontend to get URL but when I upload the file with multer, it automatically changes the file type on the backend.

I used word changes here because the file type shows correct on the frontend. For example: the file I am uploading is of type

{
path: "2023-12-14 13-19-55.mkv"
type: "video/x-matroska"
lastModified: 1702539348430
lastModifiedDate: Thu Dec 14 2023 13:20:48 GMT+0545 (Nepal Time) {}
name: "2023-12-14 13-19-55.mkv"
size: 17555107
}

But on the backend it shows:

{
  fieldname: 'imageData',
  originalname: '2023-12-14 13-19-55.mkv',      
  encoding: '7bit',
  mimetype: 'application/octet-stream',
  destination: 'uploads/',
  filename: '1712392840374-2023-12-14 13-19-55.mkv',
  path: 'uploads\\1712392840374-2023-12-14 13-19-55.mkv',
  size: 17555107
}

Why does it change the file type?

Contributor guide

Tech stack
nodejsexpress
Domain
backendapi
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
needs investigation
Prerequisites
Node.js basicsmulter middlewareHTTP file uploads
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Reproduce the issue with a minimal example using multer and an MKV file. Check if the frontend correctly sends the MIME type in the multipart form data. Investigate multer's source code to see how it determines the mimetype, especially the `FileFilter` and `DiskStorage` functions. Look for any configuration that might override the original MIME type. Consider if the issue is caused by the browser or a missing library for detecting MIME types on the server side.