Only allow certain file extensions

[SamuelJForrest]

Only allow the following file types:

• .jpg
• .png
• .svg

Make sure to update the form text to reflect this, as .svg is currently missing

Additionally, add a message/alert to the user to let them know what went wrong.

[SamuelJForrest]

I've now updated the file extensions text on the upload form, and added in alerts when an incorrect file type is selected