KFileItemListProperties::mimeType() returns an empty string if the
selected files have different types, but KMimeTypeTrader does not
support empty mimetypes as query argument.
This causes the following warning
KMimeTypeTrader: mimeType "" not found
and breaks the bluedevil's 'Send via Bluetooth' plugin.
If we know the user selected only files, we can use the default mimetype
as fallback.
BUG: 393710
FIXED-IN: 5.47