diff --git a/phpBB/includes/functions_upload.php b/phpBB/includes/functions_upload.php
index ae04fe737c..f179b2fd70 100644
--- a/phpBB/includes/functions_upload.php
+++ b/phpBB/includes/functions_upload.php
@@ -232,9 +232,9 @@ class filespec
 	{
 		if ($this->mimetype_guesser !== null)
 		{
-			$mimetype = $this->mimetype_guesser->guess($filename);
+			$mimetype = $this->mimetype_guesser->guess($filename, $this->uploadname);
 
-			if (empty($this->mimetype) || $mimetype !== 'application/octet-stream')
+			if ($mimetype !== 'application/octet-stream')
 			{
 				$this->mimetype = $mimetype;
 			}
diff --git a/phpBB/phpbb/mimetype/guesser.php b/phpBB/phpbb/mimetype/guesser.php
index 773a1f822a..c019cb5b07 100644
--- a/phpBB/phpbb/mimetype/guesser.php
+++ b/phpBB/phpbb/mimetype/guesser.php
@@ -115,17 +115,42 @@ class guesser
 			return false;
 		}
 
+		$mimetype = 'application/octet-stream';
+
 		foreach ($this->guessers as $guesser)
 		{
-			$mimetype = $guesser->guess($file, $file_name);
+			$mimetype_guess = $guesser->guess($file, $file_name);
 
-			// Try to guess something that is not the fallback application/octet-stream
-			if ($mimetype !== null && $mimetype !== 'application/octet-stream')
-			{
-				return $mimetype;
-			}
+			$mimetype = $this->choose_mime_type($mimetype, $mimetype_guess);
 		}
 		// Return any mimetype if we got a result or the fallback value
-		return (!empty($mimetype)) ? $mimetype : 'application/octet-stream';
+		return $mimetype;
+	}
+
+	/**
+	 * Choose the best mime type based on the current mime type and the guess
+	 * If a guesser returns nulls or application/octet-stream, we will keep
+	 * the current guess. Guesses with a slash inside them will be favored over
+	 * already existing without slashes. However, any guess that will pass the
+	 * first check will always overwrite the default application/octet-stream.
+	 *
+	 * @param	string	$mime_type	The current mime type
+	 * @param	string	$guess		The current mime type guess
+	 *
+	 * @return string The best mime type based on current mime type and guess
+	 */
+	protected function choose_mime_type($mime_type, $guess)
+	{
+		if ($guess === null || $guess == 'application/octet-stream')
+		{
+			return $mime_type;
+		}
+
+		if ((strpos($mime_type, '/') === false || $mime_type == 'application/octet-stream') && strpos($guess, '/') !== false)
+		{
+			$mime_type = $guess;
+		}
+
+		return $mime_type;
 	}
 }