Attachment Upload Controller:

public with sharing class AttachmentUploadController {
 
  public Attachment attachment {
  get {
      if (attachment == null)
        attachment = new Attachment();
      return attachment;
    }
  set;
  }
 
  public PageReference upload() {
 
    attachment.OwnerId = UserInfo.getUserId();
    attachment.ParentId = '0010E00000CxfKm';//Record Id the file is attached to
    attachment.IsPrivate = true;
    attachment.ContentType = 'image/jpeg';
 
    try {
      insert attachment;
    } catch (DMLException e) {
      ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,'Error uploading attachment'));
      return null;
    } finally {
      attachment = new Attachment(); 
    }
    ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,'Attachment uploaded successfully'));
    return null;
  }
} 

Attachment Upload Visualforce Page:

<apex:page controller="AttachmentUploadController">  
	<apex:sectionHeader title="Example" subtitle="Attachment Upload Example"/>
	<apex:form>
	<apex:pageMessages />
		<apex:pageBlock title="Upload a Attachment">
			<apex:pageBlockButtons >
				<apex:commandButton action="{!upload}" value="Save"/>
			</apex:pageBlockButtons>
			<apex:pageBlockSection showHeader="false" columns="2">
				<apex:pageBlockSectionItem >
					<apex:outputLabel value="File Name" for="fileName"/>
					<apex:inputText value="{!attachment.name}" id="fileName"/>
				</apex:pageBlockSectionItem>
				<apex:pageBlockSectionItem >
					<apex:outputLabel value="File" for="file"/>
					<apex:inputFile value="{!attachment.body}" filename="{!attachment.name}" id="file"/>
				</apex:pageBlockSectionItem>
			</apex:pageBlockSection>
		</apex:pageBlock>
	</apex:form>
</apex:page>  

Use enctype="multipart/form-data" in apex:form tag, when your form includes any apex:inputfile element.

<apex:form enctype="multipart/form-data">