Error "Can't find first part boundary" when uploading large file using browsers

[dangngoctau]

Reproduce steps:
I can upload a small text file and some additional data like this:

Request headers
Content-Length:827
Content-Type:multipart/form-data; boundary=----WebKitFormBoundarydBssixBaEaaAkZii
Host:secure-backend.osapps.dev-eu.peaxlabs.ch

Request Payload
------WebKitFormBoundarydBssixBaEaaAkZii
Content-Disposition: form-data; name="meta"

{"category":"standard","type":"DOCUMENT","documentDate":"2017-04-01","title":"Ihr Dokument","contentLanguage":["deu","fra","ita"],"specific":{"source":"UPLOAD","receivedAt":"2009-09-28T10:00:00+02:00","contractNumber":"contractNumber","customerNumber":"customerNumber"}}
------WebKitFormBoundarydBssixBaEaaAkZii
Content-Disposition: form-data; name="file"; filename="abc.json"
Content-Type: application/octet-stream

------WebKitFormBoundarydBssixBaEaaAkZii--

But I can't upload a pdf file like this:

Request headers
Content-Length:1760296
Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryXffIBA19WqQvxARB

Request Payload
------WebKitFormBoundaryXffIBA19WqQvxARB
Content-Disposition: form-data; name="meta"

{"category":"standard","type":"DOCUMENT","documentDate":"2017-04-01","title":"Ihr Dokument","contentLanguage":["deu","fra","ita"],"specific":{"source":"UPLOAD","receivedAt":"2009-09-28T10:00:00+02:00","contractNumber":"contractNumber","customerNumber":"customerNumber"}}
------WebKitFormBoundaryXffIBA19WqQvxARB
Content-Disposition: form-data; name="file"; filename="1000 Quick Writing Ideas.pdf"
Content-Type: application/pdf

------WebKitFormBoundaryXffIBA19WqQvxARB--

Could you please help me check the reason? Thanks!