Inline attachment are not removed during reply

In order to resolve issue #WT-298, all inline attachments are preserved in replies because they can be referenced in code. This is right for images but when other attachment (like PDF) has a wrong inline marker this will be kept during reply.

We can improve this logic by looking only for images attachments marked as inline, so other attachments will not be duplicated in subsequent replies.

Until the EML attachment that reproduces the issue is not available here, please refer to "Mail con allegato che non viene tolto in caso di risposta" email on 20-07-2020.