[mms_attachment_image] Mark non-jpeg attachments as resizable

if we have either ImageMagick or Qt available to do the resizing for us
Merge pull request #13 from monich/qt-scaling

[mms_attachment_image] Mark non-jpeg attachments as resizable
[mms_task_encode] Treat empty content type as no content type

Fixes the case if UI sends a large png file which needs to be resized AND
we are using Qt to resize it AND file name has no extension AND content type
is an empty string.