TYPO3 allows to upload files either in the backend user interface as well as in custom developed extensions. To reduce the possibility to upload potential malicious code TYPO3 uses the fileDenyPattern to deny e.g. user submitted PHP scripts from being persisted. Besides that it is possible for any editor to upload file assets using the file module (fileadmin) or changing their avatar image shown in the TYPO3 backend.
This is a companion discussion topic for the original entry at https://typo3.org/article/typo3-psa-2019-010