Why/when does iOS convert audio attachments sent using Messages into different formats?

Our app sends AIFF files as attachments in Messages using UIActivityViewController. Sometimes, when the message arrives on a mobile device or desktop machine, the file example.aiff has become example.amr or example.aiff.m4a.

I imagine that Apple is converting to a compressed format to stay under some size limits, but I can't find any documentation about the specific circumstances under which this happens. Does anyone know the details here?