Feature: #66445 - Add file extension to mimeType mapping

See forge#66445


As a fix for wrong mimeType detection for SVG files without XML prologue we added a new setting to map known file extensions to mimeTypes. The new setting is $GLOBALS['TYPO3_CONF_VARS']['SYS']['FileInfo']['fileExtensionToMimeType'] which contains an array:

        'svg' => 'image/svg+xml'


The automatic detection for mimeTypes works great, but in some special cases not. This new setting should only be used, if the automatic detection fails.