我想我要加上我的三便士价值，只是说一下如何默认的.custom-file-label和custom-file-input BS4文件输入以及如何使用它们。

后一个类在输入组上，是不可见的。而前者是可见标签，并且有一个:后伪元素，看起来像一个按钮。

<div class="custom-file">
<input type="file" class="custom-file-input" id="upload">
<label class="custom-file-label" for="upload">Choose file</label>
</div>

你不能向伪元素中添加类，但是你可以用CSS(或SASS)来设置它们的样式。

.custom-file-label:after {
    color: #fff;
    background-color: #1e7e34;
    border-color: #1c7430;
    pointer: cursor;
}

这是Bootstrap 3,4,5的解决方案。

要制作一个看起来像按钮的功能文件输入控件，你只需要HTML:

HTML

<label class="btn btn-default">
    Browse <input type="file" hidden>
</label>

这适用于所有现代浏览器，包括IE9+。如果你也需要对旧IE的支持，请使用下面所示的遗留方法。

这种技术依赖于HTML5的隐藏属性。Bootstrap 4使用以下CSS在不支持的浏览器中填充此特性。如果使用Bootstrap 3，则可能需要添加。

[hidden] {
  display: none !important;
}

旧IE的遗留方法

如果您需要IE8及以下版本的支持，请使用以下HTML/CSS:

HTML

<span class="btn btn-default btn-file">
    Browse <input type="file">
</span>

CSS

.btn-file {
    position: relative;
    overflow: hidden;
}
.btn-file input[type=file] {
    position: absolute;
    top: 0;
    right: 0;
    min-width: 100%;
    min-height: 100%;
    font-size: 100px;
    text-align: right;
    filter: alpha(opacity=0);
    opacity: 0;
    outline: none;
    background: white;
    cursor: inherit;
    display: block;
}

注意，当你点击<label>时，旧的IE不会触发文件输入，所以CSS“膨胀”做了一些事情来解决这个问题:

使文件输入的跨度为周围<span>的全宽/高 使文件输入不可见

反馈和额外阅读

我已经发布了关于这个方法的更多细节，以及如何向用户显示选中了多少文件的示例:

https://www.abeautifulsite.net/posts/whipping-file-inputs-into-shape-with-bootstrap-3/

有可接受结果的简单解决办法:

<input type="file" class="form-control">

以及风格:

input[type=file].form-control {
    height: auto;
}

尝试遵循Bootstrap v.3.3.4

<div>
    <input id="uplFile" type="file" style="display: none;">

    <div class="input-group" style="width: 300px;">
        <div  id="btnBrowse"  class="btn btn-default input-group-addon">Select a file...</div>
        <span id="photoCover" class="form-control">
    </div>
</div>

<script type="text/javascript">
    $('#uplFile').change(function() {
        $('#photoCover').text($(this).val());
    });

    $('#btnBrowse').click(function(){
        $('#uplFile').click();
    });
</script>

设置上传按钮的样式很麻烦，因为它设置的是输入的样式而不是按钮的样式。

但是你可以用这个技巧:

http://www.quirksmode.org/dom/inputfile.html

简介:

Take a normal <input type="file"> and put it in an element with position: relative. To this same parent element, add a normal <input> and an image, which have the correct styles. Position these elements absolutely, so that they occupy the same place as the <input type="file">. Set the z-index of the <input type="file"> to 2 so that it lies on top of the styled input/image. Finally, set the opacity of the <input type="file"> to 0. The <input type="file"> now becomes effectively invisible, and the styles input/image shines through, but you can still click on the "Browse" button. If the button is positioned on top of the image, the user appears to click on the image and gets the normal file selection window. (Note that you can't use visibility: hidden, because a truly invisible element is unclickable, too, and we need the to remain clickable)

／* *引导3文件样式 * http://dev.tudosobreweb.com.br/bootstrap-filestyle/ ＊ *版权所有(c) 2013 Markus Vinicius da Silva Lima *更新引导3保罗恩里克福克斯 * 2.0.0版本 *根据MIT许可证授权。 ＊ * /

(function ($) {
"use strict";

var Filestyle = function (element, options) {
    this.options = options;
    this.$elementFilestyle = [];
    this.$element = $(element);
};

Filestyle.prototype = {
    clear: function () {
        this.$element.val('');
        this.$elementFilestyle.find(':text').val('');
    },

    destroy: function () {
        this.$element
            .removeAttr('style')
            .removeData('filestyle')
            .val('');
        this.$elementFilestyle.remove();
    },

    icon: function (value) {
        if (value === true) {
            if (!this.options.icon) {
                this.options.icon = true;
                this.$elementFilestyle.find('label').prepend(this.htmlIcon());
            }
        } else if (value === false) {
            if (this.options.icon) {
                this.options.icon = false;
                this.$elementFilestyle.find('i').remove();
            }
        } else {
            return this.options.icon;
        }
    },

    input: function (value) {
        if (value === true) {
            if (!this.options.input) {
                this.options.input = true;
                this.$elementFilestyle.prepend(this.htmlInput());

                var content = '',
                    files = [];
                if (this.$element[0].files === undefined) {
                    files[0] = {'name': this.$element[0].value};
                } else {
                    files = this.$element[0].files;
                }

                for (var i = 0; i < files.length; i++) {
                    content += files[i].name.split("\\").pop() + ', ';
                }
                if (content !== '') {
                    this.$elementFilestyle.find(':text').val(content.replace(/\, $/g, ''));
                }
            }
        } else if (value === false) {
            if (this.options.input) {
                this.options.input = false;
                this.$elementFilestyle.find(':text').remove();
            }
        } else {
            return this.options.input;
        }
    },

    buttonText: function (value) {
        if (value !== undefined) {
            this.options.buttonText = value;
            this.$elementFilestyle.find('label span').html(this.options.buttonText);
        } else {
            return this.options.buttonText;
        }
    },

    classButton: function (value) {
        if (value !== undefined) {
            this.options.classButton = value;
            this.$elementFilestyle.find('label').attr({'class': this.options.classButton});
            if (this.options.classButton.search(/btn-inverse|btn-primary|btn-danger|btn-warning|btn-success/i) !== -1) {
                this.$elementFilestyle.find('label i').addClass('icon-white');
            } else {
                this.$elementFilestyle.find('label i').removeClass('icon-white');
            }
        } else {
            return this.options.classButton;
        }
    },

    classIcon: function (value) {
        if (value !== undefined) {
            this.options.classIcon = value;
            if (this.options.classButton.search(/btn-inverse|btn-primary|btn-danger|btn-warning|btn-success/i) !== -1) {
                this.$elementFilestyle.find('label').find('i').attr({'class': 'icon-white '+this.options.classIcon});
            } else {
                this.$elementFilestyle.find('label').find('i').attr({'class': this.options.classIcon});
            }
        } else {
            return this.options.classIcon;
        }
    },

    classInput: function (value) {
        if (value !== undefined) {
            this.options.classInput = value;
            this.$elementFilestyle.find(':text').addClass(this.options.classInput);
        } else {
            return this.options.classInput;
        }
    },

    htmlIcon: function () {
        if (this.options.icon) {
            var colorIcon = '';
            if (this.options.classButton.search(/btn-inverse|btn-primary|btn-danger|btn-warning|btn-success/i) !== -1) {
                colorIcon = ' icon-white ';
            }

            return '<i class="'+colorIcon+this.options.classIcon+'"></i> ';
        } else {
            return '';
        }
    },

    htmlInput: function () {
        if (this.options.input) {
            return '<input type="text" class="'+this.options.classInput+'" style="width: '+this.options.inputWidthPorcent+'% !important;display: inline !important;" disabled> ';
        } else {
            return '';
        }
    },

    constructor: function () {
        var _self = this,
            html = '',
            id = this.$element.attr('id'),
            files = [];

        if (id === '' || !id) {
            id = 'filestyle-'+$('.bootstrap-filestyle').length;
            this.$element.attr({'id': id});
        }

        html = this.htmlInput()+
             '<label for="'+id+'" class="'+this.options.classButton+'">'+
                this.htmlIcon()+
                '<span>'+this.options.buttonText+'</span>'+
             '</label>';

        this.$elementFilestyle = $('<div class="bootstrap-filestyle" style="display: inline;">'+html+'</div>');

        var $label = this.$elementFilestyle.find('label');
        var $labelFocusableContainer = $label.parent();

        $labelFocusableContainer
            .attr('tabindex', "0")
            .keypress(function(e) {
                if (e.keyCode === 13 || e.charCode === 32) {
                    $label.click();
                }
            });

        // hidding input file and add filestyle
        this.$element
            .css({'position':'absolute','left':'-9999px'})
            .attr('tabindex', "-1")
            .after(this.$elementFilestyle);

        // Getting input file value
        this.$element.change(function () {
            var content = '';
            if (this.files === undefined) {
                files[0] = {'name': this.value};
            } else {
                files = this.files;
            }

            for (var i = 0; i < files.length; i++) {
                content += files[i].name.split("\\").pop() + ', ';
            }

            if (content !== '') {
                _self.$elementFilestyle.find(':text').val(content.replace(/\, $/g, ''));
            }
        });

        // Check if browser is Firefox
        if (window.navigator.userAgent.search(/firefox/i) > -1) {
            // Simulating choose file for firefox
            this.$elementFilestyle.find('label').click(function () {
                _self.$element.click();
                return false;
            });
        }
    }
};

var old = $.fn.filestyle;

$.fn.filestyle = function (option, value) {
    var get = '',
        element = this.each(function () {
            if ($(this).attr('type') === 'file') {
                var $this = $(this),
                    data = $this.data('filestyle'),
                    options = $.extend({}, $.fn.filestyle.defaults, option, typeof option === 'object' && option);

                if (!data) {
                    $this.data('filestyle', (data = new Filestyle(this, options)));
                    data.constructor();
                }

                if (typeof option === 'string') {
                    get = data[option](value);
                }
            }
        });

    if (typeof get !== undefined) {
        return get;
    } else {
        return element;
    }
};

$.fn.filestyle.defaults = {
    'buttonText': 'Escolher arquivo',
    'input': true,
    'icon': true,
    'inputWidthPorcent': 65,
    'classButton': 'btn btn-primary',
    'classInput': 'form-control file-input-button',
    'classIcon': 'icon-folder-open'
};

$.fn.filestyle.noConflict = function () {
    $.fn.filestyle = old;
    return this;
};

// Data attributes register
$('.filestyle').each(function () {
    var $this = $(this),
        options = {
            'buttonText': $this.attr('data-buttonText'),
            'input': $this.attr('data-input') === 'false' ? false : true,
            'icon': $this.attr('data-icon') === 'false' ? false : true,
            'classButton': $this.attr('data-classButton'),
            'classInput': $this.attr('data-classInput'),
            'classIcon': $this.attr('data-classIcon')
        };

    $this.filestyle(options);
});
})(window.jQuery);