Even if the "Reset font settings" button wipes away HTML formatting, for some reasons the mail still gets sent as HTML.
To reproduce it with a very simple one-word mail:

1. Compose a new message with the word "Test"
2. Mark the text

Scenario A:
3. Don't do any formatting.
4. Send to yourself
=> received as Plain text = okay

Scenario B:
3. Don't do any formatting.
4. Reset font settings (superfluous)
5. Send to yourself
=> received as Plain text = okay

Scenario C:
3. Format bold
4. Send to yourself
=> received as HTML = okay

Scenario D:
3. Format bold
4. Reset font settings
5. Send to yourself
=> received as HTML = wrong

Scenario E:
3. Format bold
4. Unformat bold
5. Send to yourself
=> received as Plain text = okay

Scenario F:
3. Format bold
4. Unformat bold
5. Reset font settings (superfluous)
6. Send to yourself
=> received as Plain text = wrong

Obviously, "Reset font settings" doesn't reset a composed mail to Plain text, even if no formatting remains (scenario D).
Even more, if any formatting has ever been applied to the composed mail, "Reset font settings" forces the mail to be send as HTML, even if no formatting remained.