Enclosure
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Field Description:
RSS enclosure is part of the RSS 2.0 spec. It allows for the attachment of files via a URL to feed items. These files are usually in media format. It has three required attributes:  

Url:  
The URL for the file. The proper syntax for the URL is to use the fully specified address that includes the scheme.  

Length:  
The size of the file in bytes.  
 
Type:  
The MIME type of the file. Consult the tables below to determine which MIME format is acceptable.  

MIME types for iTunes  
   File        Type      
   .mp3        audio/mpeg      
   .m4a, .m4b    audio/mp4a-latm      
   .mp4        video/mp4      
   .m4v        video/x-m4v      
   .mov        video/quicktime      
   .pdf        application/pdf      
 
MIME types accept by most readers              
   File        Type      
   .mp3      audio/mpeg  
   .m4a, .m4b    audio/mp4a-latm      
   .mp4        video/mp4      
   .m4v        video/x-m4v      
   .mov        video/quicktime      
   .mpg        audio/mpeg or video/mpeg      
   .ra        audio/x-pn-realaudio      
   .mid        audio/midi      
   .wav        audio/x-wav      

Field Usage:
Including an Enclosure with your feed is optional. However, if you enter information is any Enclosure field, then you must enter proper information into all the Enclosure fields.  

Reference:
http://blogs.law.harvard.edu/tech/rss