I just found out that wildcards (*) act weird when searching for attachments.
I was searching for (cases with mails with) attachments containing some substring, say part. I started searching for attachment:part, but that didn't find anything. Apparently it doesn't look for substrings.
Then I tried attachment:"part*", as I knew the attachments' filenames started with part. That found attachments like part1.txt, but not part one.txt etc.
I was particularly interested in attachments starting with part (part plus a space). So finally I tried attachment:"part *". That did find all attachments I needed.
Now that I know about this quirk, one question remains: if I need to find part1.txt as well as part one.txt, do I really have to search for attachment:"part *" OR attachment:"part*"?!
Maybe I'm missing something...