10-a: Enable sessions.

* Using sessions seems like a more elegant way of managing state for the app. When the user uploads a file, its name and type are stored in the session. This can be accessed in subsequent routes, until the destroy route is called after the user closes the app or downloads their gif.
10-b: Create a private method for returning the filetype.

* This serves as a complement to the method which strips the filetype.
10-c: Refactor '/upload' route to set session variables for the file and type.
Change filetype validation to check an array of whitelisted file formats.
10-d: Refactor convert route to use session variables.
10-e: Refactor '/destroy' path to use session variables.
10: Remove unnecessary method: #strip_public_folder.

* Good riddance.
10: Remove filename from AJAX call.

* This is unnecessary, now that the filename is being stored in the session.
Merge branch 'refactor-with-sessions' into development

* Changed the controller to use session variables to track the user's file throughout their visit to the site.