publishToConfluence:Cannot handle embedded SVG images

[PacoVK]

This issue was raised by @faltfe (thanks!)
To keep the scope of #1206 i separated this one here:

There was the observation in another issue that publishToConfluence could not handle embdded SVG images. This can be reproduced if we set failOnMissingImages=true in docToolchian config.

To Reproduce
Steps to reproduce the behavior:

1. ensure failOnMissingImages=true in docToolchian config (this is default)
2. add the following content to the adoc file you want to publish

[plantuml,03_technical-context,svg]
....
@startuml
skinparam componentStyle uml2

foo <-u-> bar

@enduml
....

3. run publishToConfluence
4. The page will be created without attachment

Expected behavior
The attachment will be uploaded

Additional context
If we use e.g. png instead of svg it works as expected