[imageplugin] Fix MIME media type

1. The file extension might not be the correct MIME type, and is empty
   for images extracted from the clipboard
2. The image is converted to a JPEG, anyway
[imageplugin] Don't percent-encode base64 data
[imageplugin] Use QString::toLatin1() instead of QString::toAscii()
Merge pull request #6 from FlorianFieber/imageplugin

[Image Plugin] Fix MIME type and encoding