Why are mp3 files converting to wav files after being sent by text to an email.

Let me explain the issue. I received an MP3 of an original composition (not copyrighted) from a friend via text iMessage. The file is 6.1 MB in size. In looking for a way to copy it and send it to my PC, I sent it from my phone to my gmail address. When I receive the file by email, it is a WAV file and it is now 0.5 MB in size. When I play the WAV file, it is very distorted and noisy. When I go back to my iMessage, the original file still shows as a MP3 but is now 0.5 MB in size.

From this, I have a primary question: Why was the file converted from an MP3 to a WAV and then compressed when I sent it from the iMessage to my email?

My secondary questions: Why did this process compress the original MP3? Finally, how can I simply access the MP3, take it from my iPhone and save it on my PC? I hit the save feature but cannot locate it anywhere in my audio messages, files, videos, or music library.