本文的android代码测试机型为：小米4。首先用系统文件管理器选择文件，获得文件Uri，然后解析Uri得到文件真实路径。

这里借鉴的是CSDN博客里别的大佬的回答（见参考文献[1]，记得申请运行时权限 READ_EXTERNAL_STORAGE）：

//**********************************File Choose Depart****************************************

private static final String TAG1 = "FileChoose";

// 调用系统文件管理器
private void chooseFile() {
    Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
    intent.setType("*/*").addCategory(Intent.CATEGORY_OPENABLE);
    try {
        startActivityForResult(Intent.createChooser(intent, "Choose File"), CHOOSE_FILE_CODE);
    } catch (ActivityNotFoundException e) {
        Toast.makeText(this, "亲，木有文件管理器啊-_-!!", Toast.LENGTH_SHORT).show();
    }
}

private static final int CHOOSE_FILE_CODE = 0;

@Override
// 文件选择完之后，自动调用此函数
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (resultCode == Activity.RESULT_OK) {
        if (requestCode == CHOOSE_FILE_CODE) {
            Uri uri = data.getData();
        }
    } else {
        Log.e(TAG1, "onActivityResult() error, resultCode: " + resultCode);
    }

    super.onActivityResult(requestCode, resultCode, data);
}

Uri.toString()的值为
　content://com.android.fileexplorer.myprovider/external_files/AFolder/tiaojiaoqi.stl

Uri.getPath()的值为
　external_files/AFolder/tiaojiaoqi.stl

文件的实际外部存储路径为
　/storage/emulated/0/AFolder/tiaojiaoqi.stl

这里采用了StackOverflow上大神Paul Burke的代码（见参考文献[2]），在小米4 Android 8.0.0 API 26上的测试是成功的：

Uri uri = data.getData();
String sPath1 = null;
sPath1 = getPath(this, uri); // Paul Burke写的函数，根据Uri获得文件路径

/**
 * Get a file path from a Uri. This will get the the path for Storage Access
 * Framework Documents, as well as the _data field for the MediaStore and
 * other file-based ContentProviders.
 *
 * @param context The context.
 * @param uri The Uri to query.
 * @author paulburke
 */
public static String getPath(final Context context, final Uri uri) {

    final boolean isKitKat = Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT;

    // DocumentProvider
    if (isKitKat && DocumentsContract.isDocumentUri(context, uri)) {
        // ExternalStorageProvider
        if (isExternalStorageDocument(uri)) {
            final String docId = DocumentsContract.getDocumentId(uri);
            final String[] split = docId.split(":");
            final String type = split[0];

            if ("primary".equalsIgnoreCase(type)) {
                return Environment.getExternalStorageDirectory() + "/" + split[1];
            }

            // TODO handle non-primary volumes
        }
        // DownloadsProvider
        else if (isDownloadsDocument(uri)) {

            final String id = DocumentsContract.getDocumentId(uri);
            final Uri contentUri = ContentUris.withAppendedId(
                    Uri.parse("content://downloads/public_downloads"), Long.valueOf(id));

            return getDataColumn(context, contentUri, null, null);
        }
        // MediaProvider
        else if (isMediaDocument(uri)) {
            final String docId = DocumentsContract.getDocumentId(uri);
            final String[] split = docId.split(":");
            final String type = split[0];

            Uri contentUri = null;
            if ("image".equals(type)) {
                contentUri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;
            } else if ("video".equals(type)) {
                contentUri = MediaStore.Video.Media.EXTERNAL_CONTENT_URI;
            } else if ("audio".equals(type)) {
                contentUri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
            }

            final String selection = "_id=?";
            final String[] selectionArgs = new String[] {
                    split[1]
            };

            return getDataColumn(context, contentUri, selection, selectionArgs);
        }
    }
    // MediaStore (and general)
    else if ("content".equalsIgnoreCase(uri.getScheme())) {
        return getDataColumn(context, uri, null, null);
    }
    // File
    else if ("file".equalsIgnoreCase(uri.getScheme())) {
        return uri.getPath();
    }

    return null;
}

/**
 * Get the value of the data column for this Uri. This is useful for
 * MediaStore Uris, and other file-based ContentProviders.
 *
 * @param context The context.
 * @param uri The Uri to query.
 * @param selection (Optional) Filter used in the query.
 * @param selectionArgs (Optional) Selection arguments used in the query.
 * @return The value of the _data column, which is typically a file path.
 */
public static String getDataColumn(Context context, Uri uri, String selection,
        String[] selectionArgs) {

    Cursor cursor = null;
    final String column = "_data";
    final String[] projection = {
            column
    };

    try {
        cursor = context.getContentResolver().query(uri, projection, selection, selectionArgs,
                null);
        if (cursor != null && cursor.moveToFirst()) {
            final int column_index = cursor.getColumnIndexOrThrow(column);
            return cursor.getString(column_index);
        }
    } finally {
        if (cursor != null)
            cursor.close();
    }
    return null;
}


/**
 * @param uri The Uri to check.
 * @return Whether the Uri authority is ExternalStorageProvider.
 */
public static boolean isExternalStorageDocument(Uri uri) {
    return "com.android.externalstorage.documents".equals(uri.getAuthority());
}

/**
 * @param uri The Uri to check.
 * @return Whether the Uri authority is DownloadsProvider.
 */
public static boolean isDownloadsDocument(Uri uri) {
    return "com.android.providers.downloads.documents".equals(uri.getAuthority());
}

/**
 * @param uri The Uri to check.
 * @return Whether the Uri authority is MediaProvider.
 */
public static boolean isMediaDocument(Uri uri) {
    return "com.android.providers.media.documents".equals(uri.getAuthority());
}

最后根据路径打开文件，可以自由处理了。

[1] 安卓[android] 通过Uri获取File文件. https://blog.csdn.net/Gen_Ye/article/details/53537139
[2] Get real path from URI, Android KitKat new storage access framework [duplicate].
https://stackoverflow.com/questions/20067508/get-real-path-from-uri-android-kitkat-new-storage-access-framework/20559175#20559175