When using the Attachment Importer plugin, file size check errors may occur in some situations. This problem is caused by the plugin checking the file size and issuing an error if it does not match the ‘content-length’ field in the HTTP header.
To solve this problem, comment out the following code in approximately line 288 of the plugin’s index.php file
// if ( isset( $headers['content-length'] ) && $filesize ! = $headers['content-length'] ) {This will disable the file size check and no longer generate errors.