Here's the simplest test that doesn't work under Robolectric : getMimeTypeFromExtension() returns null . As a result, in general, all the code that uses this class, it becomes inconvenient to test.

How to repair?

 @RunWith(RobolectricTestRunner::class) @Config(constants = BuildConfig::class, sdk = intArrayOf(21), manifest = "AndroidManifest.xml", application = RobolectricMdiaLibApplication::class, packageName = "ru.secret.id") class FooBarTest { @Test fun MimeType() { val type = MimeTypeMap.getSingleton().getMimeTypeFromExtension("pdf") assertEquals("application/pdf", type) } }

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