text/*:usename subdir useicon
but attached image is displayed.
MIME arguments are not "additive". This means that each content-type
arg spec, or filter arg spec, is separate. In what you have, only
text/* types will use icons (assuming the m2h_external'filter is
registered for text/* types).
I believe for your case, the following should do what you want:
m2h_external'filter:usename subdir useicon
This forces all content-types processed by the m2h_external'filter
to have the files put in a subdirectory with a name specified
in the header, and an icon displayed as part of the hyperlink text
to the file.
BTW, "inline" supercedes "useicon".