TYPO3 Core

You can restrict possible file extensions per field and content element in TCA, see:

https://docs.typo3.org/m/typo3/reference-coreapi/master/en-us/ApiOverview/Fal/UsingFal/Tca.html#tca-definition

There you can replace $GLOBALS['TYPO3_CONF_VARS']['GFX']['imagefile_ext'] with your list of allowed file extensions.

[GFX][imagefile_ext] is the central setting which defines which file extensions are images file extensions and thus which files can be used for thumbnail generation, etc.

I know I can restrict file extensions, but i.e. $GLOBALS['TCA']['tt_content']['image']['config'] is using [GFX][imagefile_ext] and that 's core generated....
Wouldn't it be smart to simply add the .pdf extension to the thumbnail generation regardless of the content of [GFX][imagefile_ext]?

Update:

I added this to AdditionalConfiguration.php

if (TYPO3_MODE === 'BE') {
    /*
     * This is done due to the fact that if ['GFX']['imagefile_ext'] does not contain 'pdf' no thumbnail generation
     * for pdf files is carried out...
     */

    $addExtension = 'pdf';

    if (!GeneralUtility::inList($GLOBALS['TYPO3_CONF_VARS']['GFX']['imagefile_ext'], $addExtension)) {
        $GLOBALS['TYPO3_CONF_VARS']['GFX']['imagefile_ext'] .= ',' . $addExtension;
    }
}

.... it works like a charm :-)