Gmail had added this new feature using which you can see your Spreadsheet attachments in your mail online using Google Spreadsheets app. Previously to view a spreadsheet online, there was only a single option – View as HTML.
So now you will find 3 links with the spreadsheet attachment in your Gmail mail – View As HTML | Open in Google Spreadsheets | Download
But these 3 links appear only for the spreadsheet attachments prior to Microsoft Office 2007 beta Spreadsheets as for Spreadsheets created with Microsoft Office 2007 beta are not recognized by GMail/Google Spreadsheets so far…so you can see only Download link in front of them as is shown in the image – the one with xlsx extension.
If you open the spreadsheet from GMail attachment, a little welcome message is displayed at right-bottom corner as is below –