feat: Make dc_msg_get_filename() return the original attachment filename (#4309)

It can be used e.g. as a default in the file saving dialog. Also display the original filename in
the message info. For these purposes add Param::Filename in addition to Param::File and use it as an
attachment filename in sent emails.

node/test/test.js

@@ -446,7 +446,8 @@ describe('Offline Tests with unconfigured account', function () {
     context.setChatProfileImage(chatId, imagePath)
     const blobPath = context.getChat(chatId).getProfileImage()
     expect(blobPath.startsWith(blobs)).to.be.true
-    expect(blobPath.endsWith(image)).to.be.true
+    expect(blobPath.includes('image')).to.be.true
+    expect(blobPath.endsWith('.jpeg')).to.be.true
 
     context.setChatProfileImage(chatId, null)
     expect(context.getChat(chatId).getProfileImage()).to.be.equal(