tdf#109202 Add mimetype to image element

[Michael Stahl]

On 18.12.2017 21:37, Regina Henschel wrote:
> The disadvantage is, that in case the xlink-attributes are used to 
> reference an external image, the mime-type is not available.

i don't see this as a disadvantage: the content of an ODF package cannot
make assumptions about what some URL that points outside the ODF package
refers to; dereferencing the URL might result in a different file than
when the ODF package was created, or it might be a 404 because the file
has been removed.