Hello Petter,
usally it is as easy as you describe it.
You said you have a RichTextField and your FileUpload Control. I hope it is not binded to the same field in the document, if so, the content from the richText control could overwrite the attachment (maybe)
Is your save button set up correctly?
<xp:link escape="true"
id="linkSaveComplete" test="Save">
<xp:eventHandler event="onclick"
submit="true" refreshMode="complete">
<xp:this.action>
<xp:actionGroup>
<xp:saveDocument var="document" />
</xp:actionGroup>
</xp:this.action>
</xp:eventHandler>
</xp:link>
Try the simple action Save Document, you can also give a parameter which document should be saved, otherwise he chosses the "current" document, which could be another than you wish.
Do you have more than one datasource in your XPage?