Automatic detection of subtitle mime-types

[DavidMikeSimon]

The mime-types associated with subtitle files (e.g. "text/srt" for .srt files) are not well-known, and so are unlikely to be provided by many uploading clients. Ayamel should guess mimetypes for such files based on the extension if the type is not otherwise specifically provided. Otherwise Ayamel player will ignore them, since it uses the mimeType field to determine how to process the subtitles file.