Why I can't upload big files in File Manager?

This is caused by PHP settings "upload_max_filesize" and "post_max_size", which defaults to "2M" (2MB). You must change these values to increase the size of the files you are able to upload.
Add the following line to the file ".htaccess" on your website root directory: 

php_value upload_max_filesize 10M 

php_value post_max_size 10M

Or change these values in your php.ini file.
If memory limit is enabled by your PHP configure script, then the memory_limit also affects file uploading.

Technical FAQ

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