It’s a little hard to tell what’s going on here without seeing your settings. I don’t think it has anything to do with the post length.
We deliberately strip email signatures because they are annoying and waste space. If you don’t want that to happen you can change this setting.
Regarding attachments, do you know that they were trying to attach allowed file types?
And do you know how big the attachment was?
This guide may also be of use: Remove extraneous content from incoming emails