Spectacle can only save in PNG format. I would prefer to have an option to store screenshots as JPG too. Additionally, it is currently impossible to save screenshots with just a number tag (1.png, 2.png, ... 35.png) but the file has to have some prefix. It is a major regression in comparison to KSnapshot.
Fixed since version 16.12 which uses the KDE file dialogs when running on Plasma. *** This bug has been marked as a duplicate of bug 369174 ***