Thanks for the post and trying to get this issue resolved but I don't know if I agree that it's just an APP issue.
I'm sure it's a compatibility issue at its core but I was having issues sending edited images/screenshots as text attachments. However, if I tried to upload an edited image through the Fleep app for example, it would convert the image to .plist but if I select that image and go to share --> more --> Fleep and upload it that way, it does upload correctly. It's a bit of pain, especially if I have to leave the app that I'm on, find the image and then share it, but it is a work around.
Thanks!