As far as I know oneboxing will only work on externally linked PDFs, not uploaded ones.
With CSS you could do something like
a.attachment[href$=".pdf"] {
color: red;
&:before {
// The attachment icon is added using a pseudo-selector
// so you'd need to edit its styles here
}
}
This applies styles to links with the class attachment
only if the href
ends in .pdf