Gii generator for MongoDB Active Record created.
Docs about MongoDB gii generator usage added
mimeTypes Validator is wrong

I'm find error

```php

Wrong
 } elseif (!empty($this->mimeTypes) && !in_array(FileHelper::getMimeType($file->tempName), $this->mimeTypes, true)) {

Correct
} elseif (!empty($this->mimeTypes) && !in_array(FileHelper::getMimeType($file), $this->mimeTypes, true)) {
```

Not need call '$file->tempName' because getMimeType get object $_FILE => $file, so use $file and validator works fine
Update FileValidator.php
Fixes #2314: Gii model generator does not generate correct relation type in some special case
Merge pull request #3785 from marciocamello/patch-2

mimeTypes Validator is wrong
Merge pull request #3778 from klimov-paul/mongodb-gii

Enh: MongoDB model gii generator
Fixes issue #3740 for horizontal layouts
Doc comments and code style for 'debug' extension adjusted
Merge pull request #3789 from mikehaertl/master

Fixes issue #3740 for horizontal layouts
Fixed translation tool and docs
Added notes about composer to translation docs
Fixed translation docs typo
Removed unnecessary symlink