Remove redundant mimetype information

[bmarty]

I created element-hq/element-web#17145 because the spec is not clear.

Then I created element-hq/element-android#3273 on Element Android to remove the usage of the redundant mimetype.

But it appears that those fields are expected by Element iOS. For instance the image does not appear in the timeline of those fields are missing.

What iOS should do (IMO):

• Remove the field MXEncryptedContentFile.mimetype
• Ensure that image or any attachment sent without this field are correctly handled in the timeline and anywhere else.

I tried to fix the problem by myself on iOS but I'm really not an efficient iOS developer :/

The Android PR is waiting for iOS to be robust with the changes it includes before being merged.