Templates almost work for all content types

[RogerHaase]

Create or modify an svg image adding a tag of template. Then create an new svg image and select the former svg image as the template. When the svg image is saved, a new file is created in /wiki/data/default/data with a length of 0 bytes.

There may be similar failures for other non-text content types.

Although using templates for non-text items may have very limited usage, it is probably easier to fix it than to document the limitations and write code giving nice error messages.