digitally signed emails - signature invalid

hi ! I have problems with the configuration of my digital signature in thunderbird . i m missing something ... the problem is that when i attach sometghing to the email i m writing (html signature from thunderbird setting or any file ) and sign it with my digital signature , the regived email has an invalid signature because the content was modified . when i send an email without attachems and sign , the signature is valid . the bublic key is added in certificates in mozila , the root is added , the sent emails without attachment have a valid signature ... so the only problem is the attachment