Hey jakudo,
Thanks for using Apple Support Communities! I understand you're running into an issue when trying to copy/paste attachments within Mail. I definitely want to make sure you're able to copy and paste smoothly.
To help us get you pointed toward a resolution, I do have a couple of questions. Would you mind answering the following?
You mentioned you're able to copy the attachment without issue, so that's great. When you go to paste it, what happens? If you open a new email, and press and hold does the menu appear with options like select/select all and the option to paste isn't there, or are you not seeing that menu at all?
Does this behavior only happen in Mail? Are you able to copy and paste within other apps without issue?
What model iPhone and iOS version are you working with? You can find the iOS version in Settings > General > About.
Thanks again, and have a great day!