OPDS Flattening (#1904)

* Flattening OPDS Structure

# Changed
- Flattened OPDS structure to reduce user taps.

* Fixing format

* Fixing book series titles

* Optimized file size to use pre-calculated data to avoid an I/O touch.

* Fixes #1898 by aligning all content headers to the correct MIME types

* Remove dead code

* Fixed a bug with continue point where it fails on chapters or volumes tagged with a range

---------

Co-authored-by: Robbie Davis <robbie@therobbiedavis.com>

API/Services/DownloadService.cs

@@ -4,6 +4,7 @@ using System.IO;
 using System.Linq;
 using API.Entities;
 using Microsoft.AspNetCore.StaticFiles;
+using MimeTypes;
 
 namespace API.Services;
 
@@ -47,7 +48,7 @@ public class DownloadService : IDownloadService
                 ".zip" => "application/zip",
                 ".tar.gz" => "application/gzip",
                 ".pdf" => "application/pdf",
-                _ => contentType
+                _ => MimeTypeMap.GetMimeType(contentType)
             };
         }