Disable attachment support in xdg-email (fixes CVE-2020-27748)
Fix for #177 (CVE-2020-27748) and #205. It introduces a breaking change by removing --attach support. However as discussed in #177, the removed feature was:
- only working for thunderbird;
- not always functional (even for thunderbird) depending on the desktop environment used;
- the source of several vulnerabilities.