Add extensions to convert mimeTypes

[Goooler]

Follow up 8ba5d0f

Full mimeTypes ref: https://github.com/zhanghai/MaterialFiles/blob/19ada387f0c34484d9fd1c4403c2db41f201e743/app/src/main/java/me/zhanghai/android/files/compat/MimeTypeMapCompat.kt

[Goooler]

internal fun String.extension2MimeType(): String? {
  return MimeTypeMap.getSingleton().getMimeTypeFromExtension(lowercase()) ?: allFileTypes[this]
}

[Goooler]

internal fun String.mimeType2Extension(): String? {
  return MimeTypeMap.getSingleton().getExtensionFromMimeType(lowercase())
    ?: allFileTypes.filterValues { it == this }.keys.firstOrNull()
}