Pull Request for Issue #27494 (comment).

Requires PR #29895 for the language strings.

Summary of Changes

This Pull Request (PR) is a back port of PR #27665 from 4.0-dev.

It adds a check if the selected file is too big to be uploaded with respect to the PHP limits to the "Upload & Install Package" tab of the Extensions Installer.

If the user tries to upload a file which is too big, the upload is rejected with an appropriate browser alert message.

Testing Instructions

Requirements:
PR #29895 for the language strings.

Procedure:

1. Apply the patch of this PR and also the patch of PR #29895 .
2. Go to administrator/index.php?option=com_installer&view=install and there to tab "Upload Package File".
3. Drag to the drag and drop area a file which is too big compared to the maximum upload size shown below the file selection.
   Hint: This can be any file, it doesn't necessarily have to be a valid extension package.
   Result: See screenshot in section "Expected result" below. A browser alert is shown telling that the file can't be uploaded because it's too big. The upload is not started.
4. Select a file which is too big, using the "Or browse for file" button.
   Result: Same as in previous step. A browser alert is shown telling that the file can't be uploaded because it's too big.
5. Drag to the drag and drop area a file which is not too big compared to the maximum upload size shown below the file selection.
   Result: Uploading of not too big package file using drag and drop still works as well as without this PR.
6. Finally select another package file which is not too big, using the "Or browse for file" button, and verify that uploading still works.
   Result: Uploading of not too big package file using the "Or browse for file" button still works as well as without this PR.

Actual result BEFORE applying this Pull Request

• If post_max_size is big enough but upload_max_size is violated:
  

• If upload_max_size is big enough but post_max_size is violated:
  There is no error shown in backend if display errors is off in PHP config.
  In the PHP log file (if log_errors is switched on in PHP config):
  PHP Warning: POST Content-Length of 24829262 bytes exceeds the limit of 16777216 bytes in Unknown on line 0, referer: https://www.joomla.vmkubu02.vmnet2.local/administrator/index.php?option=com_installer

Expected result AFTER applying this Pull Request

Broswer alert when trying to upload too big file:

Documentation Changes Required

None.