Updated deprecated function from GuzzleHttp used on DropboxFile

[Pixelao]

Updated deprecated function from GuzzleHttp used on DropboxFile

The function \GuzzleHttp\Psr7\mime_type_from_filename(string $filename) is deprecated and will be removed in guzzlehttp/psr7:2.0

Use MimeType::fromFilename instead.

More info: https://docs.aws.amazon.com/aws-sdk-php/v3/api/function-GuzzleHttp.Psr7.mimetype_from_filename.html

Changes:

• Use MimeType::fromFilename instead mime_type_from_filename on function Dropboxfile->getMimetype()