Components, Admin Tools, Web Application Firewall, Configure WAF. Click on Active Request Filtering. Find "Uploads scanner (UploadShield)" and set it to No. Click on Save & Close.
Please note that if you still have an issue uploading files then it's not coming from Admin Tools. Joomla! 3.4.1 and later also includes uploads filtering and has it always enabled, without a switch to disable it. Developers have been given instructions to modify their extensions to allow uploads of files that would trigger the protection. If you are using a component which doesn't allow your users to upload certain files please contact its developer and ask them for a version updated for Joomla! 3.4. The change is trivial.
Nicholas K. Dionysopoulos
Lead Developer and Director
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