Core

Problem:
Thumbnails can not be cached by browser.

There is always the current timestamp appended to the thumbnail URL, IMHO that is not required because the thumbnails URL contains already a check sum ("t3lib_div::shortMD5()") based on the:
- $GLOBALS['TYPO3_CONF_VARS']['SYS']['encryptionKey']
- filename
- filetime

Solution:
- Remove the parameter "dummy=timestamp" from the URL
- The used md5 should be enlarged to full 32 char.

Note:
The tumbnails are cached well in the "typo3temp/" folder, that is based on an md5 sum.

The generated URL look like:
typo3/thumbs.php?&file=__ABSOLUTE_PATH_TO_FILE__&size=56x56&md5sum=d509bbc84d&dummy=1257502882

Global variable is set to the current time stamp:
typo3_src/t3lib/config_default.php:484:$EXEC_TIME = time();

Used: =======================
t3lib/class.t3lib_befunc.php:1808: $url = $thumbScript.'?&dummy='.$GLOBALS['EXEC_TIME'].$params;
t3lib/class.t3lib_befunc.php:1837: $url = $thumbScript.'?&dummy='.$GLOBALS['EXEC_TIME'].$params;

ypo3/sysext/cms/tslib/class.tslib_content.php:4619: $icon = 't3lib/thumbs.php?dummy='.$GLOBALS['EXEC_TIME'].'&file='.rawurlencode('../'.$theFile).$thumbSize.$md5sum;

typo3/show_item.php:427: $url = $thumbScript.'?&dummy='.$GLOBALS['EXEC_TIME'].$params;
typo3/class.file_list.inc:536: $href = $this->backPath.$this->thumbScript.'?&dummy='.$GLOBALS['EXEC_TIME'].$params;

And a lot of more .....
(issue imported from #M12490)