Attachment API File Upload
"content" needs to be a string if using that - it represents the actual attachment contents. What you probably want is the move-file one. If you're at /civicrm/api in the explorer click on the Code Docs tab at the top and pick Attachment for the Entity, then at the bottom of the text that appears is an example, just yours is slightly different since it's a custom field.

Also some assorted samples in here, e.g. https://github.com/civicrm/civicrm-core/blob/5.14/tests/phpunit/api/v3/AttachmentTest.php#L91