Force a zip tool to look at the xlsx file, for xlsx is a zip archive of many files. Many windows explorer version will open it if you rename it whatever.zip ! You need to filter the listing of the zip for usable file types. You do not want to text-filter any images! Most of the internal files are xml is text. I see vml and rels files labeled as XML by PKZIP. You will see the patterns around URLs. Almost any text tool can extract them: awk, sed, PERL. You can get a list of internal files of interest from a unzip list and tell unzip to extract and pipe them to stdout, where you filter out the URLS.