Issue 1376

Fixed an error where the user could upload nothing, causing the server to throw an error. Also added a check to see if the file being uploaded is a .csv file. In case no file is uploaded, flash a warning on the spreadsheet page saying "No file selected". When a non .csv file is uploaded, will flash a warning saying "You did not upload a .csv file." to the user. 

![fail](https://f.cloud.github.com/assets/6404591/2298938/4734855a-a0c4-11e3-9b72-88ce131b4998.png)

The reason this issue occurred before was because the code attempted to access an attribute of the selected file before checking if a file was even selected/uploaded.