Ignore my original response. I see you're using the Xfinity Connect mobile app. Testing myself on my Droid Turbo 2, My Xfinity Connect app was able to attach and send .jpg, .pdf, .vzm and .txt files. What make/model device are you having issues with?
I am getting the "File upload failed: "new." message when I try to attach a Word or PDF file. Just started today. How do I fix this??