Mail Attachment name randomly becomes ATT00001.bin

Hi,
I have an application that sends an email with a generated file attached. Code goes like this:
Mail.Attachments.Add(new Attachment(someMemoryStream, someFileName));
Where "someFileName" is just a constant string. Now, randomly the attachment's name becomes ATT00001.bin. I haven't figured a pattern yet when this happens. Any ideas?

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