Document sending only as citation

Hi,
last days I changed my mail program on Mac to Thunderbird and set Thunderbird as custom mail program. If I want to send a pdf document in Devonthink via right mouse click, this opens a mail window of Thunderbird. But instead of pasting the pdf there are only some lines of citation from the content of this pdf file, but not the file. This makes no sense.
In Apple Mail this wirks perfectly. Where can I change this?
Mac M2 Pro Ventura 13.6.5
Thunderbird 115.8.1 (actual)