feat: Allow admin to override the maximum file upload size

Original configuration had a hard-coded limit of 20MB per file supplied to the platform. 

This change will retain the existing 20MB limit as default, but provide the admin with the ability to override the value via the MFE_CONFIG API (getConfig)

The new function includes validation to check that the override value supplied is in fact a valid and positive integer and reverts to default 20MB in case of validation failure

Author: MuPp3t33r

src/constants.js

@@ -1,3 +1,5 @@
+import { getConfig } from '@edx/frontend-platform';
+
 export const DATE_FORMAT = 'MM/dd/yyyy';
 export const TIME_FORMAT = 'HH:mm';
 export const DATE_TIME_FORMAT = 'YYYY-MM-DDTHH:mm:ss\\Z';
@@ -52,7 +54,16 @@ export const DECODED_ROUTES = {
   ],
 };
 
-export const UPLOAD_FILE_MAX_SIZE = 20 * 1024 * 1024; // 100mb
+// FilesUpload page - Default max size: 20MB else use env override if exists and valid number
+const DEFAULT_UPLOAD_FILE_MAX_SIZE = 20 * 1024 * 1024; // 20 MB
+
+export const getUploadFileMaxSize = () => {
+  const config = getConfig();
+  const overrideMaxFileSizeMB = parseInt(config.OVERRIDE_UPLOAD_FILE_MAX_SIZE_IN_MB, 10);
+  return !Number.isNaN(overrideMaxFileSizeMB) && overrideMaxFileSizeMB > 0
+    ? overrideMaxFileSizeMB * 1024 * 1024
+    : DEFAULT_UPLOAD_FILE_MAX_SIZE;
+};
 
 export const COURSE_BLOCK_NAMES = ({
   chapter: { id: 'chapter', name: 'Section' },