Here's the scenario.... Your Android device presents you with a choice of apps to complete an action with. You're supposed to select the app and then select either "Just Once" or "Always." Either way the app opens to complete the action.
If you selected "Just Once" you'll have to choose again the next time you need to complete that same action. That might seem annoying. But what if it's the wrong choice? If you choose "Always" you won't get to choose again, at least not easily. And if the app cannot complete the action successfully, what then?
According to many lame blog posts and webzine articles, it's "easy" to change your mind after choosing "Always." They say to just press the app's "Clear Defaults" button. Here's one such article on AndroidCentral.
Fortunately AndroidCentral is swarming with brilliant users who are much more sophisticated than the folks who write the articles. I say "fortunately" because when I needed to change the option of using Photos for downloading an image file attachment, the article's advice was useless because Photos' "Clear Defaults" button was greyed out.
A user called Siddhartha Gupta suggested that to remove Photos as the default app, it's necessary to change the setting for Google+. That didn't work, either.
A user called ChromeJob pointed out that there's "a 'reset app preferences' in the action overflow button. This appears to reset ALL default app choices at once." If only it were that easy for me. It didn't work.
The thing that finally worked for me was to revert to the factory version of Photos by pressing the "Uninstall Updates" button. After doing that, I was presented with a choice of two other apps to use. The idea to alter Photos' version came to me after reading a comment byGrillMouster who stated that "when any of those apps receive an update the ["Just Once" / "Always"] dialog will pop up again, even if you had previously set an app as the default."
If you want the very gory details of why Photos didn't work, please read on to find out what I was trying to do.
I had used Firefox to log in to an Exchange Server using an Outlook Web Access (OWA) client hosted on a web page. I then opened an e-mail that had a JPG file attached to it. When I attempted to download the attachment, OWA again wanted me to provide my username and password. Photos was unable to handle that request (and neither was Firefox). It was only by completing the action with ES File Explorer File Manager that I found out about the need to provide username and password. I've included this detail in case there are others who are having trouble downloading attachments with OWA.