Recently I have spent a lot of time writing functional documents, which translates into producing tons of MS Word pages and Excel Spreadsheets. And since everybody knows how boring it is, the best thing you can do is re-use docs you have already written. Although it can save you a considerable amount of time, it could be very annoying having an attached spreadsheet with a caption that reminds everybody that you are being lazy.
To change it, right-click on the Object and select Convert option:
Then select "Change Icon" and change the Caption to the one you want. Be aware that even if it looks greyed-out, you can still edit the Caption. God bless MS!