diff --git a/doc/filesystem.md b/doc/filesystem.md
index 6ee097513..d3dbebf5f 100644
--- a/doc/filesystem.md
+++ b/doc/filesystem.md
@@ -105,6 +105,28 @@ Opens a file. `path` should be an absolute path starting with a slash
 (e.g. `/dir/filename.txt`). `mode` is a string specifying access mode. It can be
 one of "r", "w", "a", "r+", "w+", "a+". Meaning of these modes is the same as
 for `fopen` C function.
+       
+       r      Open text file for reading.  The stream is positioned at the
+              beginning of the file.
+
+       r+     Open for reading and writing.  The stream is positioned at the
+              beginning of the file.
+
+       w      Truncate file to zero length or create text file for writing.
+              The stream is positioned at the beginning of the file.
+
+       w+     Open for reading and writing.  The file is created if it does
+              not exist, otherwise it is truncated.  The stream is
+              positioned at the beginning of the file.
+
+       a      Open for appending (writing at end of file).  The file is
+              created if it does not exist.  The stream is positioned at the
+              end of the file.
+
+       a+     Open for reading and appending (writing at end of file).  The
+              file is created if it does not exist.  The initial file
+              position for reading is at the beginning of the file, but
+              output is always appended to the end of the file.
 
 Returns *File* object. To check whether the file was opened successfully, use
 the boolean operator.