(I hope this comes out as wishlist... I haven't tried it before)

As the subject says, I think squid should be able to accept
mime types in refresh_pattern. (I am surpised that this hasn't
been already implemented, maybe it is, however the documentation
seems really terrible, so I may have missed it).

For instance, I would like to be able to say that the minimum cache time
for html files (which often are sent without a last-modifies header) is
7 days. I would like to be able to do something like[1]:

refresh_pattern        text/html      0 20% 10080

(Note: [1] this may require a different command to "refresh_pattern")

However, the only way I have found to do the above it like this:

refresh_pattern         \.html$ 0 20% 10080
refresh_pattern         \.htm$  0 20% 10080
refresh_pattern         /$      0 20% 10080

(not tested yet). This is unreliable as it assumes anything ending
in .html, .htm, and / are the only html files that can exist.

If it was possible, I would put in rules for other file types, too,
eg application/x-debian-package, and image/* for instance.