if i restore my iphone from an old back-up will i lose my current files and data (eg photos)?

Yes, all content from your phone will be wiped and set back to the point in time when the backup had been taken.

But, you can transfer your photos and other files to your computer before you start.

This article explains the way to import the photos to your computer:

Import photos and videos from your iPhone, iPad, or iPod touch - Apple Support

Other files can be mailed to your mail address, or copied to another device via AirDrop, for example.

Contacts, calendars, reminders, etc, are part of iCloud and the info will sync back as soon as you set up the phone again with the same Apple ID and log into iCloud again.