Log attachment blocked events (#3615)

Include both a log entry and the blocked mime type in the gitea log when
an attachment upload is blocked.

Chosen log level is info; this may need to be dialed down to trace.

routers/repo/attachment.go

@@ -55,6 +55,7 @@ func UploadAttachment(ctx *context.Context) {
 	}
 
 	if !allowed {
+		log.Info("Attachment with type %s blocked from upload", fileType)
 		ctx.Error(400, ErrFileTypeForbidden.Error())
 		return
 	}