I'm getting an error when sending image files to the emulator.

We've pulled Emulator build 3.5.26 due to this and a few other issues. We will be releasing a recovery build later today. For the moment, downgrading to 3.5.25 is the suggested mitigation. Sincere apologies!
I have the same issue today, just after updating the emulator! BTW, the issue occured also with the embeded web chat control.
Error message :
Uncaught TypeError: this.props.sendFiles is not a function C:\Users\saibi\AppData\Local\botframework\app-3.5.26\resources\app\node_modules\botframework-webchat\built\Shell.js 26 20 {}
Apologies! We're working to make a fixed build available. In the meantime downgrading to 3.5.25 will restore this functionality.
Fixed in 3.5.27
I'm facing a similar issue, when trying to upload a WAV: [err-client] Uncaught TypeError: this.props.sendFiles is not a function C:\Users\ezequ\AppData\Local\botframework\app-3.5.28\resources\app\node_modules\botframework-webchat-speech\built\Shell.js 27 20 {}
3.5.29 is now available with a fix for the file upload issue: https://github.com/Microsoft/BotFramework-Emulator/releases
Most helpful comment
Fixed in 3.5.27