In IE 8 I choose the destination e-mail, the subject and the file to send.
When the progress bar finishes, the page get stuck and I can see an error icon (exclamation mark icon) on the browser status bar.
When I click that icon I can see 4 error lines with the same message:
hexadecimal number expected
hexadecimal digit expected
( I translate it from the Italian sentence "Prevista cifra esadecimale")
No problems with FF 3.6.x and Chrome, even with file sizes of hundreds of MB.