i want to send a selected file to a webservice, by clicking on a created button in alfresco.
Respectively i want to create a SOAP/WSDL-Message first, when the button is clicked and then the file should be attached to this SOAP/WSDL-Message. Anybody an idea how to do that?
This sounds like an Action that is invoked by a UI Action. That UI Action could exist in Alfresco Explorer, Alfresco Share, or both.
An Action is implemented using Java and has access to the full foundational Alfresco API, so you can do anything you want, including grab content and stick it in a SOAP envelope.
If you need help learning how to implement a custom action and then configuring it in the user interface, take a look at this tutorial.