This is probably becuase ".dat" files are not supported as mail attatchements in the iOS:
Viewable Document Types
.jpg, .tiff, .gif (images); .doc and .docx (Microsoft Word); .htm and .html (web pages); .key (Keynote); .numbers (Numbers); .pages (Pages); .pdf (Preview and Adobe Acrobat); .ppt and .pptx (Microsoft PowerPoint); .txt (text); .rtf (rich text format); .vcf (contact information); .xls and .xlsx (Microsoft Excel)
Ok it's not supported. So, I am assuming that since it is not supported iOS does not know what to do with it when I try and forward the email including the attachment. In this case then the native mail app should tell the user that the email cannot be forwarded including the attachement, as the attachment is not supported and give the option to send the email without the attachment. The native mail app does not respond back with anything and the assumption is made that the email actually got sent but in reality it did not. Slightly annoying.
Also, viewing the email (in gmail) using a browser on a computer the attachment name is the actual name of the PDF and not winmail.dat.
Therefore, would this be a scenario that needs a quick fix by Apple to look at the attachments before allowing forwarding?
I was having problems receiving PDF attachments in e-mail on my iPhone 5 with iOS 6.0.2.
PDF attachments would arrive as winmail.dat files.
Until a couple of months ago, I had been successfully receiving PDF attachments for several years (iPhone 3, 4 and 5).
I had one of the e-mail senders delete my contact information in their Outlook address book and the problem was fixed for any new e-mails that person sent to me (not the previous emails).
I saw this solution over in the OS X forum.
Thanks for the feedback. Unfortunatly I have no control over the persons contact info for myself on their computer. I receive PDFs on my iPhone 5 all the time without any problems. Looks like RTF messes up the iPhone native mail program. I hope Apple fixes the forwarding to give a warning message saying the email cannot be forwarded as the attachement is not recognized.