In order to increase the size limit for attachment files in PrestaShop, follow the steps below:

Step 1:  Increase file upload size limit in PHP-Apache.

Step 2: Increase file upload size limit in PrestaShop Back Office.

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PrestaShop 1.6

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PrestaShop 1.7

1. Log in to your PrestaShop Back Office. 
2. Navigate to side menu > Advanced Parameters > Administration > Upload Quota.
   
3. Change the Maximum size for attachment (in megabytes) to a value that is equal to or larger than 16 MB.
   
4. Click Save.