Saving plot to file requires a file extension

[scott-eddy]

Right now saving a plot to a file requires the user to put .png, .jpg, etc in the filename.

Additionally if the user puts something that is not supported (E.G. no file extension or a bad one such as ".txt") there is no warning or error, the dialog goes away but no file is created.

Default extensions should be selected for this functionality for a user

[facontidavide]

fixed