filenamefull path to the file starting from the SD card root directory. This function can't create a new file in non-existing directory.
modesupported mode strings are:
"r"read access. File must exist beforehand. The read pointer is located at the beginning of file. This is the default mode if is omitted.
"w"write access. File is opened or created (if it didn't exist) and truncated (all existing file contents are lost).
<file object>if file was successfully opened.
nilif file could not be opened.