Large attachments are blocked by default by Microsoft Dynamics CRM for security reasons. To allow large attachments, you need to configure IIS to accept large amounts of streamed data. This allows HTTP connections to remain OPEN longer, which may open the door to Denial of Service (DOS) attacks. This said, it is reasonable to configure IIS to go beyond the default 4 MBs limit if your server is not exposed to the internet, or is otherwise protected.
With the default 4 MBs limit, if you add large attachments to CRM records, or track emails / tasks / appointments with large attachments, you may receive errors. One example is shown below:
To enable large attachments, you need to modify both IIS and CRM settings. To modify IIS settings, navigate to your web.config file, and edit property maxRequestLength. For more information about this property, see http://msdn2.microsoft.com/en-us/library/e1f13641(vs.71).aspx. Keep in mind that the value is in KBs, with a default of 4096 (4 MBs). Here is an example of a modified web.config file where the allowed default size was doubled:
To modify CRM settings, navigate to the CRM server registry key, and edit DWORD value maxuploadfilesize. Keep in mind that the value is in bytes, with a default of 5242880 (5 MBs). Also make sure you enter data in the right format (hexadecimal vs. decimal) when editing the registry key. Here is the registry key path:
Once these changes are in place, you should be able to upload larger attachments to CRM. Make sure you only allow what you really need to go through, though!