[Typo3-dev] Extension backup and import error

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My Typo3 is installed on windows NT.
The problem is: 
In the extension manager, when chosing one extension to backup, it can be downloaded as a file, but when import this file to the EM, it reports "No extension key in file! Strange¡­.¡±. Checking the file "typo3/mod/tools/em/index.php¡± line 1256 , function ¡°extBackup¡±, I add some lines as:
			if (intval($this->CMD["doBackup"])==1)	{
				$file_path="c:/".$filename;
				$fp=fopen($file_path,"w");
				fwrite($fp,($backUpData));
				fclose($fp);...
to write the file diretly onto the disk. This time such a file can be imported correctly. Checking these two files( donwloaded in the EM and writen directly) i found their sizes are some different. the file written diretly is some larger( it is larger than the size got from strenlen($backUpData)). So I read the written file back again to $backUpData and download it as a file. Now it can be accepted as the correct format.

So who know what's reason?? 
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