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Edit: Sniffer.php

<?php
/**
 * SimplePie
 *
 * A PHP-Based RSS and Atom Feed Framework.
 * Takes the hard work out of managing a complete RSS/Atom solution.
 *
 * Copyright (c) 2004-2016, Ryan Parman, Sam Sneddon, Ryan McCue, and contributors
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification, are
 * permitted provided that the following conditions are met:
 *
 * 	* Redistributions of source code must retain the above copyright notice, this list of
 * 	  conditions and the following disclaimer.
 *
 * 	* Redistributions in binary form must reproduce the above copyright notice, this list
 * 	  of conditions and the following disclaimer in the documentation and/or other materials
 * 	  provided with the distribution.
 *
 * 	* Neither the name of the SimplePie Team nor the names of its contributors may be used
 * 	  to endorse or promote products derived from this software without specific prior
 * 	  written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS
 * AND CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
 * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 * @package SimplePie
 * @copyright 2004-2016 Ryan Parman, Sam Sneddon, Ryan McCue
 * @author Ryan Parman
 * @author Sam Sneddon
 * @author Ryan McCue
 * @link http://simplepie.org/ SimplePie
 * @license http://www.opensource.org/licenses/bsd-license.php BSD License
 */

/**
 * Content-type sniffing
 *
 * Based on the rules in http://tools.ietf.org/html/draft-abarth-mime-sniff-06
 *
 * This is used since we can't always trust Content-Type headers, and is based
 * upon the HTML5 parsing rules.
 *
 *
 * This class can be overloaded with {@see SimplePie::set_content_type_sniffer_class()}
 *
 * @package SimplePie
 * @subpackage HTTP
 */
class SimplePie_Content_Type_Sniffer
{
	/**
	 * File object
	 *
	 * @var SimplePie_File
	 */
	var $file;

/**
	 * Create an instance of the class with the input file
	 *
	 * @param SimplePie_Content_Type_Sniffer $file Input file
	 */
	public function __construct($file)
	{
		$this->file = $file;
	}

/**
	 * Get the Content-Type of the specified file
	 *
	 * @return string Actual Content-Type
	 */
	public function get_type()
	{
		if (isset($this->file->headers['content-type']))
		{
			if (!isset($this->file->headers['content-encoding'])
				&& ($this->file->headers['content-type'] === 'text/plain'
					|| $this->file->headers['content-type'] === 'text/plain; charset=ISO-8859-1'
					|| $this->file->headers['content-type'] === 'text/plain; charset=iso-8859-1'
					|| $this->file->headers['content-type'] === 'text/plain; charset=UTF-8'))
			{
				return $this->text_or_binary();
			}

if (($pos = strpos($this->file->headers['content-type'], ';')) !== false)
			{
				$official = substr($this->file->headers['content-type'], 0, $pos);
			}
			else
			{
				$official = $this->file->headers['content-type'];
			}
			$official = trim(strtolower($official));

if ($official === 'unknown/unknown'
				|| $official === 'application/unknown')
			{
				return $this->unknown();
			}
			elseif (substr($official, -4) === '+xml'
				|| $official === 'text/xml'
				|| $official === 'application/xml')
			{
				return $official;
			}
			elseif (substr($official, 0, 6) === 'image/')
			{
				if ($return = $this->image())
				{
					return $return;
				}

return $official;
			}
			elseif ($official === 'text/html')
			{
				return $this->feed_or_html();
			}

return $official;
		}

return $this->unknown();
	}

/**
	 * Sniff text or binary
	 *
	 * @return string Actual Content-Type
	 */
	public function text_or_binary()
	{
		if (substr($this->file->body, 0, 2) === "\xFE\xFF"
			|| substr($this->file->body, 0, 2) === "\xFF\xFE"
			|| substr($this->file->body, 0, 4) === "\x00\x00\xFE\xFF"
			|| substr($this->file->body, 0, 3) === "\xEF\xBB\xBF")
		{
			return 'text/plain';
		}
		elseif (preg_match('/[\x00-\x08\x0E-\x1A\x1C-\x1F]/', $this->file->body))
		{
			return 'application/octet-stream';
		}

return 'text/plain';
	}

/**
	 * Sniff unknown
	 *
	 * @return string Actual Content-Type
	 */
	public function unknown()
	{
		$ws = strspn($this->file->body, "\x09\x0A\x0B\x0C\x0D\x20");
		if (strtolower(substr($this->file->body, $ws, 14)) === '<!doctype html'
			|| strtolower(substr($this->file->body, $ws, 5)) === '<html'
			|| strtolower(substr($this->file->body, $ws, 7)) === '<script')
		{
			return 'text/html';
		}
		elseif (substr($this->file->body, 0, 5) === '%PDF-')
		{
			return 'application/pdf';
		}
		elseif (substr($this->file->body, 0, 11) === '%!PS-Adobe-')
		{
			return 'application/postscript';
		}
		elseif (substr($this->file->body, 0, 6) === 'GIF87a'
			|| substr($this->file->body, 0, 6) === 'GIF89a')
		{
			return 'image/gif';
		}
		elseif (substr($this->file->body, 0, 8) === "\x89\x50\x4E\x47\x0D\x0A\x1A\x0A")
		{
			return 'image/png';
		}
		elseif (substr($this->file->body, 0, 3) === "\xFF\xD8\xFF")
		{
			return 'image/jpeg';
		}
		elseif (substr($this->file->body, 0, 2) === "\x42\x4D")
		{
			return 'image/bmp';
		}
		elseif (substr($this->file->body, 0, 4) === "\x00\x00\x01\x00")
		{
			return 'image/vnd.microsoft.icon';
		}

return $this->text_or_binary();
	}

/**
	 * Sniff images
	 *
	 * @return string Actual Content-Type
	 */
	public function image()
	{
		if (substr($this->file->body, 0, 6) === 'GIF87a'
			|| substr($this->file->body, 0, 6) === 'GIF89a')
		{
			return 'image/gif';
		}
		elseif (substr($this->file->body, 0, 8) === "\x89\x50\x4E\x47\x0D\x0A\x1A\x0A")
		{
			return 'image/png';
		}
		elseif (substr($this->file->body, 0, 3) === "\xFF\xD8\xFF")
		{
			return 'image/jpeg';
		}
		elseif (substr($this->file->body, 0, 2) === "\x42\x4D")
		{
			return 'image/bmp';
		}
		elseif (substr($this->file->body, 0, 4) === "\x00\x00\x01\x00")
		{
			return 'image/vnd.microsoft.icon';
		}

return false;
	}

/**
	 * Sniff HTML
	 *
	 * @return string Actual Content-Type
	 */
	public function feed_or_html()
	{
		$len = strlen($this->file->body);
		$pos = strspn($this->file->body, "\x09\x0A\x0D\x20\xEF\xBB\xBF");

while ($pos < $len)
		{
			switch ($this->file->body[$pos])
			{
				case "\x09":
				case "\x0A":
				case "\x0D":
				case "\x20":
					$pos += strspn($this->file->body, "\x09\x0A\x0D\x20", $pos);
					continue 2;

case '<':
					$pos++;
					break;

default:
					return 'text/html';
			}

if (substr($this->file->body, $pos, 3) === '!--')
			{
				$pos += 3;
				if ($pos < $len && ($pos = strpos($this->file->body, '-->', $pos)) !== false)
				{
					$pos += 3;
				}
				else
				{
					return 'text/html';
				}
			}
			elseif (substr($this->file->body, $pos, 1) === '!')
			{
				if ($pos < $len && ($pos = strpos($this->file->body, '>', $pos)) !== false)
				{
					$pos++;
				}
				else
				{
					return 'text/html';
				}
			}
			elseif (substr($this->file->body, $pos, 1) === '?')
			{
				if ($pos < $len && ($pos = strpos($this->file->body, '?>', $pos)) !== false)
				{
					$pos += 2;
				}
				else
				{
					return 'text/html';
				}
			}
			elseif (substr($this->file->body, $pos, 3) === 'rss'
				|| substr($this->file->body, $pos, 7) === 'rdf:RDF')
			{
				return 'application/rss+xml';
			}
			elseif (substr($this->file->body, $pos, 4) === 'feed')
			{
				return 'application/atom+xml';
			}
			else
			{
				return 'text/html';
			}
		}

return 'text/html';
	}
}

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