Fix loading of embedded svg graphics.

This fixes loading of draw:image of the type svg in case the mimetype is not provided as in the document
create by LO attached to the bug report. When the mime type is not found we try to guess it by the content
of the file. To do that we read the first 16K data and pass it to KMimeType. After we found out the mime
type we also set it so it is there for later usage.

BUG: 303587
REVIEW: 105589