Add "preferRawImage" parameter to image attachment endpoint

[julien-nc]

This can help https://github.com/nextcloud/text/pull/2924

The logic was to return a preview if possible and the raw content as a fallback.

• If the new parameter is true, we return the raw image (only if it is supported by browsers)
• If it's false, we do as before: try preview, raw as fallback

#getImageAttachmentUrl() has a new related parameter.