Core

According to Steffen Ritter, the references property should be able to take in a comma separated list of sys_file_reference UID's (currently not documented, see also http://forge.typo3.org/issues/34137).

But the following TS produces an exception:
10 = FILES
10 {
references = 188,189
renderObj = TEXT
renderObj.data = file:current:publicUrl
}

#1316889798: uid of record has to be numeric

InvalidArgumentException thrown in file
/var/www/vhosts/_SHARED_T3_SRC/6.0.x/typo3_src-git-master/t3lib/file/Repository/FileRepository.php in line 240.

t3lib_file_Repository_FileRepository::findFileReferenceByUid("188,189")

Adding the following TS line resolves the issue:
references.listNum = 0

Btw: As documented, it works for files, e.g.:

10 = FILES
10 {
files = 1,2,3
renderObj = TEXT
renderObj.data = file:current:publicUrl
}

--> Multiple paths to files are outputted.