Update snapchat_agent.php

[acdameli]

separate isMedia into isImage and isVideo functions which can be used independently to identify if the media is an image or a video. The isMedia function relies on the individual functions and allows a consumer of this class to use these headers which are much more accurate than the media_type attribute which I have (anecdotally) found to be unreliable.

You seem to prefer the more verbose:

if (test) {
  return TRUE;
}
return FALSE;

pattern, I didn't look very hard for contribution guidelines so I went with the a condensed style. Let me know if you have a strong preference for the other style.

[Voiceeeeee]

You need to change line 180
return isImage($data) || isVideo($data);
To
return $this->isImage($data) || $this->isVideo($data);
Or else you'll get an undefined function error.