since updating to Catalina , I cannot upload any kind of attachments in Mail

How can you upload documents of any kind as attachments in a Mail message? Since Catalina all the attachments that I try to send are not being uploaded. I cannot send any message with an attachment. When trying this window content pops-up This message contains attachments that are still being loaded.

Even if I wait for a long time no message can be sent. This is valid for small attachments of few kb or bigger ones.