OJS detects MIME types of files using one of three methods, depending on what's available: first PHP's fileinfo extension, then PHP's mime_content_type function, then the server's "file --brief --mime [filename]" command. Only files with "text/plain" or "text/css" types are accepted. These three methods tend to use the same technique in determining file types, called a "magic file" (see http://linux.die.net/man/5/magic
for details). Investigate your phpinfo() to see which of the above three methods is likely to be used, and then investigate that method's configuration (http://php.net
has lots of documentation) to find out e.g. what magic file it's using. That file might need updating.
Public Knowledge Project Team