tdf#109202 Add mimetype to image element

Hi Samuel, hi Thorsten, hi all,

I try to bring the requested mime-type attribute to ODF. In the 
discussion in the TC it was suggested, to make such attribute not an 
attribute of the <draw:image> element but to make it an attribute of the 
<office:binary-data> element.

I have thought about that proposal and see these advantage/disadvantages:

The advantage would be, that the mime-type would be also available for 
other places, where a <office:binary-data> element is used, without 
adding it to all these places individually. Such elements are 
<text:list-level-style-image> or <draw:fill-image> for example.

The disadvantage is, that in case the xlink-attributes are used to 
reference an external image, the mime-type is not available. [In case 
the xlink-attributes refer to an image inside the package, the mime-type 
is available via the manifest. In case of flat file format, the image is 
always base64encoded into a <office:binary-data> element.]

Any other arguments?

What do you think about the problem?

https://cgit.freedesktop.org/libreoffice/core/commit/?id=4469b29faeb8dbf7793a5d81d9c5ddebacf3015f
https://bugs.documentfoundation.org/show_bug.cgi?id=109202
https://issues.oasis-open.org/browse/OFFICE-3943
https://lists.oasis-open.org/archives/office/201712/msg00025.html

Kind regards
Regina