PreviewJob: Add file extension to thumbnail temp files
Previously, when previewing remote files, kio would create a local copy
of the file using QTemporaryFile's default template. This would result
in filenames, like /tmp/dolphin.XXXXXX. However, some thumbnail plugins,
like kdegraphics-thumbnailers' libkdcraw thumbnailer, require the file
extension to be valid in order to produce a preview.
This commit updates the temp file's naming template to include the original file extension (if any).