fix uploadService arrow functions 'this'

in 31d472e, the reader.onprogress, reader.onerror, and reader.onload functions here were changed from regular `function` declarations to arrow functions.

Usually this makes no difference in the operation of the code, but there is one exception.
Arrow functions don't get their own `this`, they just inherit it from the scope. When I swapped this out and made it an arrow function, I didn't notice it used `this`.

In this instance, `this` was the same thing as `reader`, which we already have a reference to. So it's an easy swap.

www/js/control/uploadService.ts

@@ -58,13 +58,13 @@ function readDBFile(parentDir, database, callbackFn) {
             };
 
             reader.onerror = (error) => {
-              logDebug('Error while reading file ' + JSON.stringify(this.error));
-              reject({ error: { message: this.error } });
+              logDebug('Error while reading file ' + JSON.stringify(reader.error));
+              reject({ error: { message: reader.error } });
             };
 
             reader.onload = () => {
-              logDebug('Successful file read with ' + this.result?.['byteLength'] + ' characters');
-              resolve(new DataView(this.result as ArrayBuffer));
+              logDebug(`Successful file read with ${reader.result?.['byteLength']} characters`);
+              resolve(new DataView(reader.result as ArrayBuffer));
             };
 
             reader.readAsArrayBuffer(file);