Use mb_detect_encoding() instead of finfo()

The Fileinfo functions are not installed by default on Windows, so use a different method to determine whether the stream is valid or binary.

src/Smalot/PdfParser/PDFObject.php

@@ -187,13 +187,12 @@ public function cleanContent(?string $content): string
 
         // Now that all strings and dictionaries are hidden, the only
         // PDF commands left should all be plain text.
-        // Detect MIME-type of the current string and prevent reading
+        // Detect text encoding of the current string to prevent reading
         // content streams that are images, etc. This prevents PHP
         // error messages when JPEG content is sent to this function
         // by the sample file '12249.pdf' from:
         // https://github.com/smalot/pdfparser/issues/458
-        $finfo = new \finfo(\FILEINFO_MIME);
-        if (false === strpos($finfo->buffer($content), 'text/plain')) {
+        if (false === mb_detect_encoding($content, null, true)) {
             return '';
         }