email: Fix inline images sent in old HTML markup
Support inline images without a filename

Such images are attached to emails by Lotus Notes. Technically, these images
are valid attachments, although historically osTicket would reject them
Merge pull request #708 from greezybacon/issue/661

email: Fix inline images sent in old HTML markup

Reviewed-By: @enricoandreoli
Reviewed-By: @sgozzi