Below are examples of how to specify external viewers:

# Use xv if X is running
image/*;                xv %s ; test=test -n "$DISPLAY";

text/x-csrc;            view %s; needsterminal

# Various multimedia files
audio/mpeg;             xmms '%s'; test=test -n "$DISPLAY";
application/pdf;        xpdf '%s'; test=test -n "$DISPLAY";
application/postscript; ps2ascii %s ; copiousoutput