Fix bug #382437 "Regression in kdialog causes wrong file extension"

Summary:
BUG: 382437
One cause of the bug is commit at plasma-integration:
0dafb9403266d6064074250d44b74dc0db946cfb
Make sure we always set a default mime filter in save mode
KFileWidget::setMimeFilter() documentation says a default mime filter
should be set if the dialog is being open in save mode.
Without a default filter, the dialog uses a custom item (created by
concatenating all the mime type filters) as default. This is not useful
because it means an additional click for the user to select a filter that can
actually be useful.
If the initiallySelectedMimeTypeFilter() is empty, we use the first mime
filter as fallback.

Chrome opens kdialog with 2 mime types: application/octet-stream (means any file) and (for example) image/jpeg. But application/octet-stream is associated with *.bin extension. When you choose this mime extention becomes *.bin. That commit does this mime default (it's first).
This patch blocks changing extention to bin for "application/octet-stream" selected.

Test Plan: Tested with Google Chrome. Used Open/Save dialogs for files.

Reviewers: Plasma, dfaure

Reviewed By: dfaure

Subscribers: ngraham, aacid, broulik, plasma-devel, Frameworks

Tags: Frameworks, Plasma

Differential Revision: https://phabricator.kde.org/D10114