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

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?