I suspect it's a group setting.?
To what what group admin options there are, I just created a new group, and there is a group email setting with these options:
1) Plain text
2) HTML -- No attachments
3) HTML -- Include attachments as links
4) HTML -- Include attachments in emails?
I'll hazard a guess that #2 is set, ie, the Felder forum is configured to strip attachments, but the certain inline attachments make it through.
Mark
