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Sitecore automatically generates thumbnails for all files uploaded to the Media Library, except for files which match the ignorefiles mask defined in the web.config file. 

<ignorefiles>

  <file find="icon16x16"/>

  <file find="icon32x32"/>

  <file find="icon48x48"/>

  <file find="temp"/>

  <file find="_thumb"/>

</ignorefiles>

In case a file's name matches one of the masks, the physical file will be uploaded, but the media Item and the thumbnails will not be created.