Extension won't install
Some extensions won't install on Joomla 3.4.0 giving an error of "No file selected. Unable to find install package"
No file selected. Unable to find install package
(Note that this error is very generic and can apply to many other issues. This is however the only UI error shown as a result of this issue that affects Joomla 3.4.0 ONLY)
What is the cause
Extra security checks for PHP files were added for file uploads in JFile::upload. Unfortunately these were not disabled for the extension installer meaning any uncompressed zip files were interpreted as insecure and therefore not installed.
How to fix
Ensure that your zip files are properly compressed before installing
Apply the following patch https://github.com/joomla/joomla-cms/pull/6180.patch (this will be shipped in Joomla 3.4.1)