The problem of progress event when upload file

I'm trying to use the .on('progress', cb) method for listening the progress of file upload.But smothing worng hapend when I pass different type param to the .attach.The progrss is fine when pass the file path, but event.percent shows 100% and it took minutes before uploading to server when pass Buffer.

Relevant versions:

superagent: 6.1.0
@types/superagent: 4.1.10
typescript: 4.1.2
ts-node: 9.0.0

Pass a file path: (the file total size is 82.3MB)

const filePath = 'path/to/the/file';
superagent.post(uploadUrl)
        .attach('upload', filePath, path.basename(filePath))
       .on('progress', event =>{
               console.log(event.loaded)
       })
       .end((err, res) => {
           if(!err) console.log(res.status)
       })

result:

The loaded is increase slowly。When loaded = totalSize，upload action is finished。

Pass Buffer

const filePath = 'path/to/the/file';
superagent.post(uploadUrl)
        .attach('upload', fs.readFileSync(filePath), path.basename(filePath))
       .on('progress', event =>{
               console.log(event.loaded)
       })
       .end((err, res) => {
           if(!err) console.log(res.status)
       })

result

181
86302755
86302757
86302858
86302859
86302861
86302967
86302969
86303027
// Wait half a minute here
200

The loaded increase so fask. Loaded = totalSize and it took minutes before uploading to server.

ladjs / superagent

The problem of progress event when upload file #1610