我尝试使用标准方法来确定mime类型，但我不能使用MimeTypeMap.getFileExtensionFromUrl(uri.getPath())保留文件扩展名。这个方法返回一个空字符串。所以我做了一个重要的解决方案来保留文件扩展名。

下面是返回文件扩展名的方法:

private String getExtension(String fileName){
    char[] arrayOfFilename = fileName.toCharArray();
    for(int i = arrayOfFilename.length-1; i > 0; i--){
        if(arrayOfFilename[i] == '.'){
            return fileName.substring(i+1, fileName.length());
        }
    }
    return "";
}

并且保留了文件扩展名，可以获得如下所示的mime类型:

public String getMimeType(File file) {
    String mimeType = "";
    String extension = getExtension(file.getName());
    if (MimeTypeMap.getSingleton().hasExtension(extension)) {
        mimeType = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension);
    }
    return mimeType;
}

在kotlin中，这要简单得多。

解决方案1:

fun getMimeType(file: File): String? = 
    MimeTypeMap.getSingleton().getMimeTypeFromExtension(file.extension)

方案二:(文件扩展名函数)

fun File.mimeType(): String? = 
    MimeTypeMap.getSingleton().getMimeTypeFromExtension(this.extension)

首先，你应该考虑调用MimeTypeMap#getMimeTypeFromExtension()，就像这样:

// url = file path or whatever suitable URL you want.
public static String getMimeType(String url) {
    String type = null;
    String extension = MimeTypeMap.getFileExtensionFromUrl(url);
    if (extension != null) {
        type = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension);
    }
    return type;
}

get file object....
File file = new File(filePath);

then....pass as a parameter to...

getMimeType(file);

...here is 


public String getMimeType(File file) {
        String mimetype = MimeTypeMap.getSingleton().getMimeTypeFromExtension(MimeTypeMap.getFileExtensionFromUrl(Uri.fromFile(file).toString()).toLowerCase());
        if (mimetype == null) {
            return "*/*";
        }
        return mimetype;///return the mimeType
    }

这里没有一个答案是完美的。以下是一个结合了所有热门答案的最佳元素的答案:

public final class FileUtil {

    // By default, Android doesn't provide support for JSON
    public static final String MIME_TYPE_JSON = "application/json";

    @Nullable
    public static String getMimeType(@NonNull Context context, @NonNull Uri uri) {

        String mimeType = null;
        if (uri.getScheme().equals(ContentResolver.SCHEME_CONTENT)) {
            ContentResolver cr = context.getContentResolver();
            mimeType = cr.getType(uri);
        } else {
            String fileExtension = getExtension(uri.toString());

            if(fileExtension == null){
                return null;
            }

            mimeType = MimeTypeMap.getSingleton().getMimeTypeFromExtension(
                    fileExtension.toLowerCase());

            if(mimeType == null){
                // Handle the misc file extensions
                return handleMiscFileExtensions(fileExtension);
            }
        }
        return mimeType;
    }

    @Nullable
    private static String getExtension(@Nullable String fileName){

        if(fileName == null || TextUtils.isEmpty(fileName)){
            return null;
        }

        char[] arrayOfFilename = fileName.toCharArray();
        for(int i = arrayOfFilename.length-1; i > 0; i--){
            if(arrayOfFilename[i] == '.'){
                return fileName.substring(i+1, fileName.length());
            }
        }
        return null;
    }

    @Nullable
    private static String handleMiscFileExtensions(@NonNull String extension){

        if(extension.equals("json")){
            return MIME_TYPE_JSON;
        }
        else{
            return null;
        }
    }
}

对于Xamarin Android(来自@HoaLe的回答)

public String getMimeType(Uri uri) {
    String mimeType = null;
    if (uri.Scheme.Equals(ContentResolver.SchemeContent))
    {
        ContentResolver cr = Application.Context.ContentResolver;
        mimeType = cr.GetType(uri);
    }
    else
    {
        String fileExtension = MimeTypeMap.GetFileExtensionFromUrl(uri.ToString());
        mimeType = MimeTypeMap.Singleton.GetMimeTypeFromExtension(
        fileExtension.ToLower());
    }
    return mimeType;
}