This depends greatly on the carriers involved. At least with Verizon (in USA), simply having a PNG header and extension is enough for a file to be classified as

This depends greatly on the carriers involved. At least with Verizon (in USA), simply having a PNG header and extension is enough for a file to be classified as an image. If the "image" is 1MB or less, the image doesn't get reencoded. I took advantage of this back in the day (when I had a severely limited data plan and unlimited texting), by writing a script for my computer that would take a large file, split it into 1MB chunks, prepend a 1x1px PNG image to the chunks, and send them to my number through the email-to-MMS gateway sequentially. Then on my phone, I wrote a script that would remove the PNG headers and concatenate the chunks, giving me the original file. My workflow consisted of SSH-ing into my PC from my phone, initiating a download, and then running the downloaded file through my scripts. By doing this, I could send up to 500MB per day to my phone without using any data.