fix(ffi): Generate correct temporary file names for media attachments

[stefanceriu]

Fixes element-hq/element-x-ios#843

As the issue above describes, simple text files (and surely others) aren't currently renderable iOS side. This happens because we use mime_guess to infer an extension from the content type and that results in a list of extensions sorted in no particular order. In this case text/plain results in asm because it just happens to be the first match from this list which iOS doesn't know what to do with.

To fix that this PR switches over to mime2ext (like this comment from mime_guess suggests) which does return more commonly used extensions.

Separately, the body of an attachment is usually its original filename which we are currently not using. This PR also addresses that by adding an optional body property and trying to use it as much as possible when generating the temporary file name. This will in turn show up in the iOS system dialogs when trying to save said file.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.02 ⚠️

Comparison is base (034aa04) 76.44% compared to head (dfc20ed) 76.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1851      +/-   ##
==========================================
- Coverage   76.44%   76.42%   -0.02%     
==========================================
  Files         140      140              
  Lines       15977    15977              
==========================================
- Hits        12214    12211       -3     
- Misses       3763     3766       +3     

Impacted Files	Coverage Δ
crates/matrix-sdk/src/media.rs	58.13% <ø> (ø)

... and 1 file with indirect coverage changes

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[pixlwave]

Ok seems reasonable to me. Having thought about it, given the spec says the body should be the filename, it does seems reasonable to take that first and if it fails use the mimetype as a fallback. Let's see how it works in the real world 🤞