Cannot open email attachments (.jpg) in Mail

Here's a strange phenonomon, since I already confirmed that iPhone Mail supports jpg formats:

when receiving emails with image attachments (e.g. jpg format),

these attachments will show up at the end of each email,

but nothing happens upon hitting the download button.

Even stranger phenonomon:

when I receive the same email on my iPod Touch, I have no problem displaying the attachments

- and upon forwarding that email to myself from the iPod, the attachments are suddenly visible within the email on the iPhone.