A valid $fileId[tmp_name] is expected” In Magento 2.3.6
Here I am using localhost, for the server you still use this path to the File directory.
Step 2: Open Uploader.php.
Open Uploader php file
Please note that this is the source directory of Magento 2, to edit it you must grant administrator privileges.
Step 3: Scroll down to line 589.
Comment line 589
Replace
$this->validateFileId($fileId);
to
//$this->validateFileId($fileId);
Step 4: Save change.
Run upgrade and compile command
php -d memory_limit=12G bin/magento setup:upgrade
php -d memory_limit=12G bin/magento setup:di:compile