fix: Use getFilename() instead of the actual filename on disk (#3521)

* fix: Use msg.getFilename() instead of the file's name in some cases

* fix: Use msg.getFilename() instead of the file's name in initializeDraft()

* fix: Use msg.getFilename() instead of the file's name in MediaItem

* fix: Use the correct file name in MediaView

* refactor: `msg` param of `getManuallyCalculatedSlideInfo()` was always null

* Improve comment

* Revert "refactor: `msg` param of `getManuallyCalculatedSlideInfo()` was always null"

We will unfortunately need getManuallyCalculatedSlideInfo() with `msg`
param

This reverts commit 60e8248db32dc22812d3f23da194bab644802045.

* fix: Fix drafting images

This fixes a bug introduced in 14f69f87e8b11a7d0568e58593946a1f1c01e75e:
When you drafted an image, pressed Back, and opened the chat again, then
the height of the drafted image was wrong and tapping the image opened a
preview for the wrong image.

I do think that theoretically it would be nicer to use getSlideForMsg
here, because we already have a DcMsg, but this didn't work because a)
the width and height wasn't gotten from the msg and instead 0 was passed
and b) the code tries to save a msgId instead of the message instead,
and loading the message from the database fails later since it's just a
draft.

I didn't want to try and fix these things, because they might be bigger
refactorings and I don't know the code.

* fix: Use the original message's filemime if there is one

...instead of trying to guess the mimetype from the uri

src/androidTest/java/com/b44t/messenger/uitests/offline/SharingTest.java

@@ -93,7 +93,7 @@ public class SharingTest {
       }
     }
     Uri uri = Uri.parse("content://" + BuildConfig.APPLICATION_ID + ".attachments/" + Uri.encode(pngImage));
-    DcHelper.sharedFiles.put("/" + pngImage, 1);
+    DcHelper.sharedFiles.put(pngImage, 1);
 
     Intent i = new Intent(Intent.ACTION_SEND);
     i.setType("image/png");