jpg default extension is dropped from [Content_Types].xml

[tdittmar]

Because of the fact that default content types are stored in a map by mime-type in merge-relations-and-content-types.js/mergeContentTypes, the default "jpg" extension is dropped, since the same mime type is used for both "jpeg" and "jpg".

For this reason, images with the .jpg extension will cause an error when opening the document in Word.