checkimages: use siteinfo fileextensions for allowed formats
checkimages: use siteinfo fileextensions for allowed formats

Bug: T120080
Change-Id: I075a4cd1c020392ea2f39fc6929ef621ef1ee65f

wikimedia/pywikibot-core,darthbhyrava/pywikibot-local,jayvdb/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,hasteur/g13bot_tools_new,wikimedia/pywikibot-core,hasteur/g13bot_tools_new,jayvdb/pywikibot-core,npdoty/pywikibot,smalyshev/pywikibot-core,happy5214/pywikibot-core,happy5214/pywikibot-core,h4ck3rm1k3/pywikibot-core,magul/pywikibot-core,magul/pywikibot-core,Darkdadaah/pywikibot-core,PersianWikipedia/pywikibot-core,h4ck3rm1k3/pywikibot-core,Darkdadaah/pywikibot-core