DocumentFileExtensions.kt: Use simple string operations to preserve e…

…xtension when renaming

MimeTypeMap's getExtensionFromMimeType() and getMimeTypeFromExtension()
are not consistent with each other. Querying the extension for
`audio/mp4` returns `m4a` as expected, but querying the MIME type for
`m4a` returns `audio/mpeg`, which is associated with the `mp3`
extension.

Due to this, whenever an output file needed to be renamed, files that
originally had the `m4a` extension would get changed to `mp3`. This
commit fixes the issue by removing the whole extension -> MIME type ->
extension round trip when renaming files. Instead, it just appends
everything after the last dot from the original filename when renaming.

Fixes: #292

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

app/src/main/java/com/chiller3/bcr/DocumentFileExtensions.kt

@@ -5,7 +5,6 @@ import android.content.Context
 import android.net.Uri
 import android.provider.DocumentsContract
 import android.util.Log
-import android.webkit.MimeTypeMap
 import androidx.documentfile.provider.DocumentFile
 
 private const val TAG = "DocumentFileExtensions"
@@ -112,8 +111,12 @@ fun DocumentFile.renameToPreserveExt(displayName: String): Boolean {
             buildString {
                 append(displayName)
 
-                val ext = MimeTypeMap.getSingleton().getExtensionFromMimeType(type)
-                if (ext != null) {
+                // This intentionally just does simple string operations because MimeTypeMap's
+                // getExtensionFromMimeType() and getMimeTypeFromExtension() are not consistent with
+                // each other. Eg. audio/mp4 -> m4a -> audio/mpeg -> mp3.
+
+                val ext = name!!.substringAfterLast('.', "")
+                if (ext.isNotEmpty()) {
                     append('.')
                     append(ext)
                 }