Cannot send email with attachment via api

[wk-as1]

I'm trying to send email with attachment via api, as below:

EMAIL=test@test.pl&SUBJECT=Test&TAGS[FOO]=bar&TAGS[TEST]=example&ATTACHMENTS=[{filename:'LICENSE.txt',path:'https://raw.github.com/nodemailer/nodemailer/master/LICENSE'}]

Or using local path:
[{filename:'LICENSE.txt',path:'/var/www/mailtrain/server/files/uploaded/LICENSE.txt'}]

Response is: {}
No email received. Nothing in logs.
Email is correctly sent and received when removing ATTACHMENTS field.

Please advice.

Full curl:

curl --location 'https://host/api/templates/6/send?access_token=xxxxxx'
--header 'Content-Type: application/x-www-form-urlencoded'
--data-raw 'EMAIL=test@test.pl&SUBJECT=Test&TAGS[FOO]=bar&TAGS[TEST]=example&ATTACHMENTS=[{filename:'''LICENSE.txt''',path:'''https://raw.github.com/nodemailer/nodemailer/master/LICENSE'''}]'