Could be anything, really. The MIME type is just a suggestion from the browser. For example, you mention that "$_FILES['userfile_PFAM']['type']=='text/plain' works for files with .txt extension" - but that depends on the client sending it. If you want to check what a file really is you have to look at the file. (All characters <chr(128); isolated "\n" characters or "\r\n" pairs, etc.)
"Text file" is awfully vague, to be honest. "text/plain" is just any text file format that doesn't have any more specific text/ MIME type. In other words, anything other than text/html, text/xml, text/javascript, text/vnd.ms-word, ....