REST API calls using json param do not work in Wordpress context --backslashes are added
There is a long discussion on this on this PR but the approach still appears to be unresolved. I have closed the PR to track through gitlab https://github.com/civicrm/civicrm-core/pull/12813
The issue:
When CiviCRM receives an API request through extern/rest.php with a json parameter, and the CMS is WordPress, an error will result.
This is because Civi bootstraps WordPress to authenticate the API user, and WordPress escapes all quotation marks in $_POST variables whether or not PHP's magic_quotes_gpc is enabled. CiviCRM is not expecting an escaped string, and JSON decoding fails due to the extra backslashes. See wp_magic_quotes() which is called by wp-settings.php during bootstrap.
To upload designs, you'll need to enable LFS and have admin enable hashed storage. More information