Ordinarily, single-page PDFs and images get attached as a default View in Place. When that View in Place is grayed out, it makes me think that at some point in time, you disabled that feature with attachments.
With your Mail compose window still open with its attachments, launch the Terminal application and enter the following with a return afterward:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool false
Now, right-click on your single-page PDF or image attachments and select View in Place. Note that word-processing documents and multi-page PDFs will still attach as icons as Apple designed them to do. This View in Place is only for your visual confirmation as it has no bearing on how the recipient's Mail client will display them.