Bug: PDF option disappears after deleting an image from the stack when all are of the same type

[Nehal Sharma]

Problem:

If the user selects all png files for example, then deletes one or more of them, the pdf option disappears from the Output dropdown. This happens since the resetting of output options is wrong

Screenshot:

Solution:

I'll update the options such that pdf is always displayed, even for the yet to be implemented feature when pdf files are accepted for merging