TYPO3 Core

The difference for EXT: between 6.2 and newer versions is due to the check for EXT:@in TemplateService in 6.2 versus using @GeneralUtility::getFileAbsFileName() in 7.6 and newer. TemplateService->getFileName actually checks for file existence with file_exists while GeneralUtility::getFileAbsFileName() "only" checks using GeneralUtility::validPathStr().

I discussed this with Wouter, since we want to educate people to use extensions to provide JavaScript files and not put them in fileadmin, the current behaviour for master, 8 and 7.6 is ok.

The remaining question is whether to change the behaviour in 6.2 or not: Use the check from GeneralUtility::getFileAbsFileName() instead of the current code:

        if (substr($file, 0, 4) === 'EXT:') {
            $newFile = '';
            list($extKey, $script) = explode('/', substr($file, 4), 2);
            if ($extKey && \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::isLoaded($extKey)) {
                $extPath = \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extPath($extKey);
                $newFile = \TYPO3\CMS\Core\Utility\PathUtility::stripPathSitePrefix($extPath) . $script;
            }
            if (!@file_exists((PATH_site . $newFile))) {
                if ($this->tt_track) {
                    $GLOBALS['TT']->setTSlogMessage('Extension media file "' . $newFile . '" was not found!', 3);
                }
                return;
            } else {
                $file = $newFile;
            }
        }

I also updated the documentation not to feature examples including files from fileadmin anymore:

https://github.com/TYPO3-Documentation/TYPO3CMS-Reference-Typoscript/pull/60

Just to be sure: This ist not a problem with including external or internal files in general!

At this point, TypoScript (at least in 6.2 LTS) is not working as expected, since a parameter for a JS-file can only be used if the JS-file is external.

This works:
page.includeJS.myscript = http://SOMEDOMAIN/JS/myscript.js?parameter=value
page.includeJS.myscript.external = 1

This does not work:
page.includeJS.myscript = fileadmin/JS/myscript.js?parameter=value