SVG To PNG: Pasting functionality not checking that the file is of .svg format

[SotoFidel]

Pasting any file format other than .svg is possible, but it shouldn't. This leads to a raw string of text instead of
the expected converted .png image.

Steps to reproduce:

1. Copy an image in a format other than .svg (.png, .jpeg, etc)
2. Paste into file drop zone

Result:

[SotoFidel]

I'll be taking a crack at this issue I made. Also just noticed that it's missing the "Allows pasting images from clipboard" subtitle. I can make that a part of the PR.

[SotoFidel]

#80 Created PR