I have to upload my artwork, but it's writte; error: ActiveRecord::RecordInvalid - Validation failed: File ext is invalid (only JPEG, PNG, GIF, and WebM files are allowed, File size is too large. Maximum allowed for this file type is 0 MiB THIS IS BAD!!! What do I do?!

Are you perhaps trying to upload flash? The ability to upoad flash content was removed ~2 months ago

If not flash, what extension type is your file?