I got an e-mail this morning with nothing in the subject line, nothing in the body of the e-mail, and with an attached file. This happens occasionally. I always wonder if it's an author sending a query who doesn't know any better...but I am not curious enough to actually open that attachment. This is a common way to pass viruses and I am not going to take that chance. Last time I got a virus, my computer was in the shop for a week and they had to completely wipe and reformat the hard drive. I cannot tell you the problems that caused.
When you submit a query via e-mail, write it in your word processor. Double check for spelling and grammar, etc. Then when it's polished and ready, COPY and PASTE it into the body of your e-mail. Do not attach it. And always put "Query" somewhere in the subject line.
Now, I am aware that some publishers have a downloadable form on their website for you to fill out and return as an attachment, or they say to go ahead and send your query/submission as an attachment. If they say that, then fine. Go ahead and do it. But there are also those that say to query within the body of the e-mail, no attachments. When in doubt, take this safer route.