Fix mime type search queries

[roborourke]

ElasticPress has a bug that results in searches incorrectly filtering on exact matches for truncated mime types such as image meaning no results are returned. This corrects the use of terms and also adds prefix queries which are more performant than the built regex query.

Fixes #97

[roborourke]

I'll make an upstream issue too

[roborourke]

Upstream issue here 10up/ElasticPress#1604

[hm-backport]

The backport to v3-branch failed:

Commits ["0cd19d96c13c2e525511fe5ef0f0cff59cd43594"] could not be cherry-picked on top of v3-branch

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport v3-branch
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 0cd19d96c13c2e525511fe5ef0f0cff59cd43594
# Create a new branch with these backported commits.
git checkout -b backport-98-to-v3-branch
# Push it to GitHub.
git push --set-upstream origin backport-98-to-v3-branch
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is v3-branch and the compare/head branch is backport-98-to-v3-branch.

[hm-backport]

The backport to v4-branch failed:

Commits ["0cd19d96c13c2e525511fe5ef0f0cff59cd43594"] could not be cherry-picked on top of v4-branch

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport v4-branch
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 0cd19d96c13c2e525511fe5ef0f0cff59cd43594
# Create a new branch with these backported commits.
git checkout -b backport-98-to-v4-branch
# Push it to GitHub.
git push --set-upstream origin backport-98-to-v4-branch
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is v4-branch and the compare/head branch is backport-98-to-v4-branch.