Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 39820)
+++ wp-includes/functions.php	(working copy)
@@ -2373,6 +2373,14 @@
 		} else {
 			$mime = false;
 		}
+		if( ( $mime === 'application/octet-stream' || $mime === false ) && function_exists ( 'finfo_file' ) ){
+			/* if the mime type is false or octet stream, its possible we are dealing with an image file not supported by PHP 
+           Constants (this applies at least to svg, but might apply to other file types) 
+			To fix this, use finfo to determine the mimetype instead
+			*/
+			$finfo = finfo_open( FILEINFO_MIME_TYPE );
+			$mime = finfo_file( $finfo, $file );
+			finfo_close( $finfo );
+		}
 	} catch ( Exception $e ) {
 		$mime = false;
 	}