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What does Mail do to the file or files differently when it sends attachments that are Windows friendly?

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@bckbck - Thanks. :-) I don't know why, but I find rep a lot harder to come by here than on StackOverflow. – Moshe Jul 28 '11 at 15:24
@Moshe - that is probably because there are many fewer people viewing – Mark Jul 28 '11 at 15:28
up vote 7 down vote accepted

According to

By default, Mail assumes your recipient is also a Mac user and therefore includes the resource forks (if any) of attached files. Normally a Mac user sees such attachments as a single file, whereas a Windows user sees two individual files - one containing the data fork of the file and the other containing the resource fork.

When you choose "Windows Friendly" attachments, Mail strips the resource fork so that Windows users receive just one file, not two (one of which would be unusable anyway). In most cases - at least for files created with modern applications - all the crucial parts of files are in the data fork; as long as the filename has the correct extension and they have an appropriate application, Windows users can open the file.

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Is a resource fork that ._yourfilename file that I see on some USB drives used with Mac? – Moshe Jul 28 '11 at 15:24
Yes, resource forks may be encoded as ._yourfilename in filesystems that don't have a better way to do it. – GEdgar Jul 28 '11 at 15:40

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