Revert "Upgrade mail gem to drop dependency on mime-types."

This reverts commit 498cc15aa9.

Noticed a certain amount of extra allocations due to this upgrade,
reverting to confirm if this is the case

.gitattributes

@@ -1,14 +1,11 @@
 # Set default behaviour, in case users don't have core.autocrlf set.
 * text=auto
 
-# Treat email fixtures as binary files so CRLF are not converted to LF.
-*.eml binary
-
-# Explicitly declare text files we want to always be normalized and converted
+# Explicitly declare text files we want to always be normalized and converted 
 # to native line endings on checkout.
 *.yml text
 
-# Custom for Visual Studio, very unlikely, but lets keep it
+# Custom for Visual Studio, very unlikely, but lets keep it 
 *.cs     diff=csharp
 *.sln    merge=union
 *.csproj merge=union