[NEW FEATURE] File upload support for different file types

Background: i needed to upload file like pdf or doc, and today the only possibility we have to upload is image, so i made this for my self, that why i propose to the community.

xblock edit(studio): now in "Allow upload" you have 3 possibility (False, Image, File), then if you select file the field "Allow file ext" will appear and you can entre several extensions separate by ,(comma).

xblock student(lms): If file is selected the student will be able to upload one file and instead of display image he will have a link "Download file"