prop=imageinfo&iiprop=url|thumbmime needs iiurlwidth=

prop=imageinfo&iiprop=url|thumbmime needs iiurlwidth=

A thumb is only generated when prop=url and a urlwidth is given. Adding
a hint to param description.
list=allpages does not have a urlwidth param and therefor cannot get the
thumbmime, adding thumbmime to the filter list to remove it from output

Change-Id: Ic1dbdb9b07f6325756058d6a0aa6ea148499fdfb