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I just added two files as an attachment to an e-mail in MS Entourage 2004.
In Finder, the files have a size of 2,5 MB respectively 205 kB.

When I send them as an e-mail, Entourage tells me that the sent mail has a size of 3,5 MB!

  • Why? Does this have to do with AppleDouble encoding?
  • Is there a way to avoid that increase in size?
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2 Answers 2

up vote 2 down vote accepted

To further expand Anthony's answer (which is correct), remember than when email was created, it was intended to send text (ASCII chars). When the idea of sending binary files became a reality, the e-mail infrastructure was already in place and working and it would have been too hard to change it to support sending binary data (plus it would have added complexity).

In order to solve the problem, the solution was to MIME encode the files (as correctly pointed by @mark in the comments).

As you can guess, this makes the size bigger, but allows the binary file to be represented in ASCII. Since ASCII has only 128 chars (and some are control chars!) because it was 7 bit, the 8 bit version, called Extended ASCII added another 128 chars to the mix (allowing accents and some other tricks), but that's just another story much better explained in Wikipedia and countless sites about it :)

To create a simple analogy, imagine you have to represent the entire alphabet using only vowels. You'd start creating combinations of the 5 letters you have to represent the missing ones, so for example the letter "B" would have to be represented as "AA" (just an example), then C would be "AAA", then D could be "AE", etc.

As you can see, I'm only using vowels, but I need more than 1 to represent consonants. This is more or less in simple terms what MIME does. It uses standard ASCII chars to represent a hell lot more, thus the increase in size.

Hope this helps you understand the process that goes on when Mail (or entourage) says: "attaching…". It's encoding the file :)

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Thanks for your help. I was quite relieved as I heard from the recipient that the sent files still had the same size there. :-) However: If a mail server has a restriction concerning file size, e g. "5 MB", is this the size of the "real files" or the size of the MIME encoded message? –  Martin May 2 '11 at 18:09
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@Martin mail server restrictions are usually based upon the size of the message itself (the server really doesn't know much about MIME, it only receives and transmits bytes). The size that the mail server uses is the MIME encoded message, since that is the real size of the data that the server has to store. –  Martín Marconcini May 2 '11 at 20:31
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This isn't an Apple or Entourage issue, this is normal behaviour that happens when attachments are encoded to be sent with emails -- encoding the data to the email format makes the attachments roughly one-third heavier.

This has nothing to do with the Apple filesystem.

There is no way to stop this happening. If file size is a problem, consider uploading to an FTP server, or using a system like Box.net, YouSendIt, or DropBox to share the files -- these methods don't require encoding and will not increase the size.

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The expanding is because the files are MIME encoded en.wikipedia.org/wiki/MIME so one byte will be mapped to more –  Mark May 2 '11 at 14:36
    
@Mark: thanks for your help. I was quite relieved as I heard from the recipient that the sent files still had the same size there. :-) However: If a mail server has a restriction concerning file size, e g. "5 MB", is this the size of the "real files" or the size of the MIME encoded message? –  Martin May 2 '11 at 18:08
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