Custom file processors 

For custom needs in terms of file processing, registration of custom file processors is available.

Create a new processor class 

The file must implement the \TYPO3\CMS\Core\Resource\Processing\ProcessorInterface and two required methods.

canProcessTask()
Will decide whether the given file should be handled at all. The expected return type is boolean.
processTask()
Will then do whatever needs to be done to process the given file.

Register the file processor 

To register a new processor, add the following code to ext_localconf.php

$GLOBALS['TYPO3_CONF_VARS']['SYS']['fal']['processors']['MyNewImageProcessor'] = [
    'className' => \MyVendor\ExtensionName\Resource\Processing\MyNewImageProcessor::class,
    'before' => ['LocalImageProcessor'],
];
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With the before and after options, priority can be defined.