How are you trying to send it? I simply email the .xls or .xlsx file to my iPad and then the attachment can be opened in Numbers directly from the iPad's native Mail app.
Otherwise, this is a problem with your computer's email, and I have no idea how to fix that, though a careful examination of you email program's preferences may help.
Also, make sure you have an actual .xls or .xlsx extension on the file when you email it. A lot of modern operating systems recognize a file type without the extension internally, but when emailing the file an extension is essential.
You could also look into getting DropBox account and the DropBox app. This works well for importing files into Numbers, though you can not export them that way. Well, you can, but the service isn't free (anymore). It is called DropDAV.