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Doesn't matter how you edit MIDI files. The 3250 (like most Nokia phones) have a built-in, fixed list of file types that built-in apps (like Messaging) refuses to send (forward).
In some cases renaming the file on the device will work, but the recipient will have to be able to rename it back, too.
Note also that the MMS specifications do not support/allow for arbitrary file types to be sent as MMS (there is - or at least used to be - a, so called, MMS Conformance document on http://www.openmobilealliance.org that lists the supported attachement file types/formats for MMS).