This was the mysqldump command you ran

mysqldump --single-transaction --quick mydatabase attachments --password=abc123 -u root > d:\attachments.sql

There are two parameters you should try

extended-insert

Someone wrote this post in the MySQL Documentation

Posted by Enrico Modanese on July 13 2006 1:05pm

I often get errors [MySQL 4.* and 5.*] on reloading a dump of databases having big blobs. I found the solution disabling the --extended-insert (that comes inside the multiple option --opt, enabled by default) with --skip-extended-insert. I think this way is safer, but it is also more more slow.

hex-blob (Optional)

Dump binary columns using hexadecimal notation (for example, 'abc' becomes 0x616263). The affected data types are BINARY, VARBINARY, the BLOB types, and BIT.

SUMMARY

Your mysqldump should now look like this

mysqldump --single-transaction --skip-extended-insert --hex-blob --quick mydatabase attachments --password=abc123 -u root > d:\attachments.sql

CAVEAT

Using --skip-extended-insert will make each row be a single INSERT command. This will cause two problems

1. much slower mysqldump
2. much bigger mysqldump

Make sure Drive D: has enough space (Maybe 2TB ???)

Give it a Try !!!