Change sticker mimetype fallback to image

[surakin]

Fixes #4110

Content

Motivation and context

Screenshots / GIFs

Tests

• Step 1
• Step 2
• Step ...

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Thank you for your contribution! Here are a few things to check in the PR to ensure it's reviewed as quickly as possible:

• Your branch should be based on origin/develop, at least when it was created.
• The title of the PR will be used for release notes, so it needs to describe the change visible to the user.
• The test pass locally running ./gradlew test.
• The code quality check suite pass locally running ./gradlew runQualityChecks.
• If you modified anything related to the UI, including previews, you'll have to run the Record screenshots GH action in your forked repo: that will generate compatible new screenshots. However, given Github Actions limitations, it will prevent the CI from running temporarily, until you upload a new commit after that one. To do so, just pull the latest changes and push an empty commit.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.26%. Comparing base (5880bbb) to head (647dcc7).
Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ctories/event/TimelineItemContentStickerFactory.kt	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4111      +/-   ##
===========================================
- Coverage    83.27%   83.26%   -0.01%     
===========================================
  Files         1884     1884              
  Lines        49068    49068              
  Branches      5760     5760              
===========================================
- Hits         40862    40858       -4     
- Misses        6139     6140       +1     
- Partials      2067     2070       +3     

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[bmarty]

Thanks, make sense. Can you sign the CLA please?

[surakin]

There you go