You can create an attachment from a VF page in a more straightforward fashion.  Here's an example from my dev org:

Controller:

public class PositionExtensionController {
 
    ApexPages.StandardController stdController;
 
    public Attachment attachment {
        get {
            if (attachment == null) attachment = new Attachment();
            return attachment;
        }
 
        set;
    }
 
    public PositionExtensionController (ApexPages.StandardController stdController)
    {
        this.stdController = stdController;
    }
 
    public PageReference save() {
        attachment.parentid = stdController.getRecord().id;
        insert attachment;
       
        PageReference page = ApexPages.currentPage();
        page.setRedirect(true);
        return page;
     }
}

Page:

<apex:page standardController="Position__c" extensions="PositionExtensionController">
<apex:messages />
    <apex:form id="theForm">
        <apex:pageBlock >
            <apex:pageBlockSection >
                <apex:inputFile value="{!attachment.body}" filename="{!attachment.name}"/>
                <apex:commandButton value="Save" action="{!save}"/>
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
    <apex:pageBlock >
        <apex:repeat value="{!Position__c.attachments}" var="attachment">
            <apex:image height="200px" value="{!URLFOR($Action.Attachment.Download, attachment.Id)}"/><p/>
        </apex:repeat>
    </apex:pageBlock>
</apex:page>

As the file input is backed by fields from the attachment, its ready to go once the postback is complete and the save method is invoked.