Make sure you don't have "Process query as regular expression" checked, or the '@' character could be treated as a special symbol. As for files like .DOC files, they are a special file format and FileSeek can't properly search them. Because they aren't just plain text, you'll have to open them up in Word to search them unfortunately.

.HTM files are plain text, so FileSeek will have no problem with them. .RTF files have a lot of "control characters" in them, but your text will still be visible as plain text, so FileSeek will work fine with them as well. .DOC files however are notoriously difficult to parse, and are not even close to plain text. As a result FileSeek can't read them.