Check uploaded and listed files for mime type

[jofranz]

General

Currently we only check file endings (like docx) to verify if a file is of a current type. This sould better be handled by checking it's mime type.

Meanwhile used method to check file endings:
CRM_Civioffice_MimeType::hasSpecificFileNameExtension($file_name, CRM_Civioffice_MimeType::DOCX)

Approach

mime_content_type() can be used to check the mime type of a file.

Problems with this approach:

• mime_content_type() is unable to access uploaded files as they are in a temp folder probalby somewhere in apache. The folder (/tmp/phpHNHdUd/) where this file should be doesn't exist. However move_uploaded_file() can access/move it
• Besides the docx mimetype, this method also returns application/octet-stream or application/zip for only some files tested. We don't want to exclude these files, as they do work with our currently used unoconv converter