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I am sorry to hear that you are having an issue uploading that file. Sometimes there are styles within .xls files that prevent them from being uploaded into Constant Contact. In this case, I suggest re-saving your file as .csv rather than .xls. This should do the trick! However, if you continue to have the same issues while uploading the .csv file then I would suggest trying on a different web browser.
All of this said, I would also like to provide you with our Knowledge Base article that goes over the upload process. Click Here for that information.
I hope that this helps but please let us know if you need additional assistance. Thank you for using Constant Contact, have a great day!