Allow user to remove existing extension

[theTisch21]

This PR adds a switch to allow the user to remove the existing file extension before adding .jpg, rather than just adding .jpg to the end of it.

Without the switch:

Img123.heic -> Img123.heic.jpg

With the switch enabled:

Img123.heic -> Img123.jpg

[theTisch21]

This should also resolve #18, giving the option to replace extensions if so desired

[theTisch21]

Apologies for the delay, these past few months have been absurdly busy for me. This new commit should include all requested changes!