According to the official release:
"A privilege escalation vulnerability has been found in Contact Form 7 5.0.3 and older versions. Utilizing this vulnerability, a logged-in user in the Contributor role can potentially edit contact forms, which only Administrator and Editor-role users are allowed to access by default. This issue has been reported by Simon Scannell from RIPS Technologies.
To minimize damage from possible attacks utilizing those vulnerabilities, Contact Form 7 5.0.4 and higher will restrict the local file attachment feature. More particularly, you will no longer be able to specify an absolute file path that refers to a file placed outside the wp-content directory. You can still specify files inside the wp-content directory with relative or absolute file paths, so all you need to change is the location of the attachment files."
According to the changelog:
"Specifies the capability_type argument explicitly in the register_post_type() call to fix the privilege escalation vulnerability issue."